Answer:
b. credibility
Step-by-step explanation:
Credibility is the quality that someone has when they can be believed in. In this case, Joe has a lot of credibility in the subject of steroid use because as a professional bodybuilder, he is part of an enviroment in which steroid use is rampant.
Listeners will assume that Joe most likely knows first-hand about what he is talking about, and this will add to his credibility.