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.