Here, we need a correct comparisons structure. We're comparing "$330 toward higher education this year" to "$7 toward higher education in previous years."

A. The participial phrase "previously paying..." is an adverbial modifier. The implication here is that it modifies the entire clause before it: "students boycotted classes..., previously paying..." This does not set up an appropriate comparison structure. Incorrect.

B. "for which was previously paid" is both awkward an unnecessarily passive. "For which the students had previously paid..." would have been acceptable.

C. It is awkward to break up the construction "$330 toward higher education" with the modifier "compared to..." in the middle. "Previously" does not correctly modify "$7 per year;" it should modify the verb "required," not the amount of money.

D. Correct comparison structure.

E. "as opposed to" is a colloquial construction, and not preferred GMAT usage.