After reviewing Act V, I would have to say the answer is D. Antonio says that him and Viola have been inseparable ever since he saved her from drowning, however Orsino claims that's nonsense because Viola has been serving him ever since. This makes Antonio mad because he's in love with Viola, and suspects that she is "cheating" on him. This then leads to him finding out that Viola is actually Cesario in disguise.