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.