Answer:
A. More people enjoy role-playing games than platformer games.
Step-by-step explanation:
It is A because the genre of game is a factor of a video game.
B is trying to state an objective truth that one genre is better than another but not giving a reason (like it being more enjoyable).
C isn't even really game related, that would be a study on which gaming platforms are more enjoyable and why.
D has already been established. The current study is to determine what makes them more or less enjoyable.