What is one reason that the reader knows Montresor is an unreliable narrator?
I don't know whether C or D is the correct answer because both make sense to me.
c. He does not reveal everything about the past or
d. He is feverish and unable to think clearly.
In short, having to decide between your answers, c or d, I would choose c. However there is more to consider than that.
"The Cask of Amontillado" is narrated in the first person and Montresor, admits in the first sentence that he will have revenge. He speaks directly to the reader as "you" assuming that the reader knows his soul. Montresor's voice is so even--not strained or emotional--he tells his story straightforwardly. However, he leaves in what he wants the reader to know and also leaves out what he does not want the reader to know. This is the critical point of the first person narrative--it is manipulative. The reader cannot decide for herself or himself what to believe about a character because it is a monologue. His feelings are hidden. (That is suspicious for a person seeking revenge.) The most terrifying moment in the story is when Fortunato knows he is being sealed up. However, Montresor is calm. He says, "I had scarcely laid the first tier of the masonry when I discovered that the intoxication of Fortunato had in a great measure worn off. The earliest indication I had of this was a low mourning cry from the depth of the recess. It was not the cry of a drunken man. There was then a long and obstinate silence. I laid the second tier, and the third, and the fourth."
In this story Poe presents the material without any interpretation. What makes this story so dark and sinister is the lack of emotion of the narrator.
If there ever were a sociopath, it would be Montresor.