Question

Read this excerpt from Infinite Jest.

I cannot make myself understood. ‘I am not just a jock,’ I say slowly. Distinctly. ‘My transcript for the last year might have been dickied a bit, maybe, but that was to get me over a rough spot. The grades prior to that are de moi.’ My eyes are closed; the room is silent.

How does the narration affect the narrator’s credibility in this excerpt?
His honesty renders him earnest and genuine.
His slang makes him seem youthful and intriguing.
His French jargon makes him seem international.
His explanations depict him as a bit defensive.

Answer 1

How does the narration affect the narrator’s credibility in this excerpt?

His explanations depict him as a bit defensive.

The character is addressing a room, presumably about his transcript; he is explaining why his grades were poor. By say 'de moi', he means that the grades before that are 'of him' or his full potential: his actual grade. He is talking to a room full of people to fully explain the situation. 

Answer 2

His explanations depict him as a bit defensive.

The question is asking about his credibility or ability to be deemed as trustworthy. Option B and C should automatically be eliminated because they do not actually have anything to do with credibility. Because his explanations are being used to show how he is "not just a jock", he is coming across as defensive. He is trying to prove to his audience that even though he did cheat some, he is still capable of doing the work and succeeding as evidence by his prior grades. He doesn't want them to see him as a stereotyped dumb jock, but instead as an intelligent and capable person.