This tells us that Maria trained, and trained relatively harder than Sue. But it doesn’t tell us that Maria trained objectively hard—it’s possible that Sue didn’t do any training.

This lets us make an inference if we already know that Maria trained hard (sufficient condition); it doesn’t tell us anything if she won the sailboat race (necessary condition).

For example, if all Maria’s opponents were sick on the day of the race, then she would win the race. But if she wins the race, it doesn’t let us conclude that all her opponents were sick.

This introduces hard training as a necessary condition for beating a champion. We know that Maria beat a champion, so it must follow that she satisfied the necessary condition of training hard. (C) therefore guarantees the author’s conclusion.

This is irrelevant: we want to prove that Maria trained hard, not that Sue didn’t train hard.

This is irrelevant: the question is whether Maria trained hard for this race, not how generally fast she is compared to Sue.