He did to study for the final exam must have been ; his grade improved from a b to an a.

The correct answer is whatever.
Whatever he did to study for the final exam must have been helpful; his grade improved from a B to an A.
The point of this assignment is to add the appropriate pronoun - either which, that, so what, or whatever. Whatever is the only option that fits the example above because the other options don't make any sense. 

He eats some of the men