and to examine the life he is living and the society in which he is living it. He has been so numbed by the old woman's death that he cannot report for work the next day. Watching the technicians revive his wife makes him aware of their automatic, well practiced procedures, ones they have perfected in bring back others who no longer wanted to live in Montag's society. Montag reaches the conclusion that he, as well as those around him, are not happy, but live instead in a kind of spiritual misery that is glossed over by the superficial pleasures offered them by the state. Montag has begun to think for himself which places him in direct and dire conflict with his repressive, authoritarian government. The path and the results of his rebellion comprise the remainder of the novel.

Clarisse also asks Montag other equally thought-provoking questions.

For example, she asks him whether firemen used to put out fires instead of starting them. Her question makes Montag obviously uncomfortable. He tries to make light of her query and proceeds to claim that all houses have always been fireproof. Clarisses betrays her youthful innocence with her next statement, however.

"Strange. I heard once that a long time ago houses used to burn by accident and they needed firemen to stop the flames."

Montag laughs when he hears this, which provokes irritation from Clarisse.

Next, she asks Montag whether he has ever seen the cars racing down a particular boulevard. For his part, Montag doesn't answer the question. We see why in a minute.

Clarisse then shares that her uncle once landed in jail for two days after being caught driving 40 miles an hour on the highway. Clarisses then asks Montag what he thinks of the treatment her uncle received. To Clarisse, her uncle's arrest was both ludicrous and sad. She asks Montag whether he agrees with her assessment.

For his part, Montag diverts her with the claim that she thinks about too many things. He is obviously uncomfortable with Clarisse's line of questioning, for her questions highlight the glaring problems of their society.

Later, Clarisse asks Montag whether he has ever considered having children. She also asks him whether he has ever smelled old leaves. Her questions again highlight the lack of contentment in Montag's life. Obviously, Clarisse's questions reinforce the dysfunction in their society. People have become so detached from their natural surroundings that they no are no longer capable of responding to stimuli that isn't artificial or pre-programmed.

Clarisse refuses to accept that fact that Montag is some kind of "guardian of her life". As a fireman, he has been happy burning books because he thinks he is protecting society. However, Clarisse makes him question that belief. In addition, Clarisse asks Montag if he is happy? Montag doesn't have an immediate reply because he has never really allowed himself to think about that question, although he has been feeling uphappy about his life for quite some time. When Montag enter his house after talking to Clarisse, he finds his wife has just tried to commit suicide. This incident serves to reinforce Montag's feeling that something is wrong in his life and his search begins to find answers that will give him some kind of satisfaction.

In Ray Bradbury's Fahrenheit 451, the question Clarisse asks Montag that seems so pivotal to his life is, "Are you happy?"

Montag is a fireman by trade. The job of firemen in this futuristic society is not to put out house fires, but to start them. Their overall purpose is to destroy homes that contain books, something is not allowed in this society. Other things have taken the place of books—things that anesthetize people toward anything of importance in the world. They do not ask questions or share opinions about anything, but happily ingest the propaganda pumped into their homes through their wall televisions and the seashells ear buds they listen to as they rest at night.

Clarisse and Montag meet on the street one day and begin to have conversations. She asks him about things he has never considered. She shares images of the world as it used to be. For instance, she speaks about dew on the grass in the morning. She holds a yellow flower under Montag's chin (a practice of young children) to see if he is in love: for if the flower reflects its yellow color onto the chin, the chin's owner is in love.

At first Montag thinks Clarisse is extremely odd. However, more and more in their brief association, Montag begins to consider the things she says to him. Life takes on new meaning for him. This galvanizes him forward to ask questions about who he is and what he does, planting the seed of desire for more knowledge—a seed that takes root and grows within him.

Perhaps the question that has the greatest impact on him is one that she throws over her shoulder as she says goodnight after their walk together:

Are you happy?

This concept is completely alien to him. He hears the question and he is incredulous—asking her departing back to repeat what she said. But she is gone. His immediate response is:

Happy! Of all the nonsense. 

He stopped laughing. [...] Of course I'm happy. What does she think? I'm not?

Montag reflects on Clarisse, and how she was able to stir his mind to new ideas in the shortest bit of time. 

What incredible power of identification the girl had...

He likens her to someone watching a marionette, anticipating what the puppet's next move will be. This is especially telling in that most of the members of Montag's society are nothing but puppets, marionettes. They are manipulated to think and act as society wishes.

As Montag ponders Clarisse's question, and then faces his wife's attempted suicide, along with emerging doubts he now has about things he had never before considered, he says...

I don't know anything anymore.

This is the beginning of a new life for Montag. It seemed that he was asleep for the longest time, but Clarisse quickly provides him with ideas, images and questions that change the way he sees himself and the world in which he was once so comfortable. It does not take long for him to realize that he is not happy, and with that knowledge he searches for something of value in himself, books and in society—or in the quiet rebellion surging beneath society's façade of satisfaction and personal fulfillment.

I assume, based on the words you tagged, the question to which you refer is asked in the first meeting, early in the novel, between Guy Montag and a teenage girl named Clarisse. Montag is a fireman, but in this dystopian, science-fiction novel a fireman may not be the heroic figure we presume firemen to be today. In fact, Montag is the protagonist of Fahrenheit 451 by Ray Bradbury, a fire-starter who burns books, not a man who extinguishes fires and saves lives.

When the novel begins, Montag is leaving  his work and heading home one night. He is unaccountably and surprisingly met by a seventeen-year-old girl named Clarisse McLellan. She is a rather extraordinary young girl, and she has a lot of things to say and a lot of questions to ask as she walks along with Montag.

One of the questions she asks is a simple question, though it could get both of them in trouble. She asks:

"Do you ever read any of the books that you burn?"

Montag is a a bit taken aback by her question, just as he is by the entire conversation they are having. Nevertheless, he answers her.

Montag replies laughing, "that's against the law!" 

His answer is correct. He is a government employee and he knows it is against the law to read books. His answer is perfectly appropriate.

His tone is also correct. With his laugh, he implies that even thinking about reading a book is a ridiculous and therefore impossible notion for a fireman whose job it is to destroy them.

Montag indicates with this answer that he is content with his job and has been for his past ten years of service. While that may be true (for now) about his job, we soon learn that he is not as content in his personal life or with the world he is living in. 

During his conversation with Clarisse, Montag says just the right thing to maintain his position as an official government fireman.

For more interesting and insightful analysis of the classic Bradbury novel, see the excellent eNotes sites linked below. 

Her parting question is one that changes his entire perspective on his life.  She asks, as they are parting after their first meeting, "Are you happy?"  At first, he laughs the question off and thinks to himself, "Of course I'm happy!"  Of all of the things to ask.  After all, didn't the novel open up with the words "It was a pleasure to burn," and then go on to describe Montag's firece joy at burning books?  And then he meets this girl, who is different, who asks probing questions, who seems like a breath of fresh air, and she tosses that question at him right as they part.  Montag considers her question "nonsense."

By the end of the evening however, he has to seriously reconsider his initial rejection of her question.  As he steps into his cold, empty house, he remember the book he has hidden behind the grille; why would he have that if there wasn't some part of him that wasn't content with life as it was.  As he goes into the bedroom and feels the emptiness there too, pretty soon his security in his own happiness fades quickly.  He realizes, quite suddenly,

"He was not happy.  He was not happy.  He said the words to himself.  He recognized this as the true state of affairs.  He wore his happiness like a mask and the girl had run off across the lawn with the mask."

Clarisse, with her question, has forced himself to face the fact that he wasn't happy at all. He lives an empty life, with a wife that is so miserable that she tries to kill herself, but in total denial over that misery, and he doesn't know where to go from there.  Her question launches him into a journey of self-discovery and questioning that will change his life for good.  I hope that helps a bit; good luck!