My answer is
(B). It took me 50 seconds. The time spent would have been much reduced if the underlined part had been corrected to end at "a larger amount" rather than "a larger amount
of students".
Also, "The university’s board of overseers
have" or "The university’s board of overseers
has"? I think "has" should be used in GMAT. It is good to be strict / rigid in actual GMAT test.
"not only X but also Z" requires strict parallelism between X and Y. Strict rules are actually fantastic because we can eliminate those answers that violate rules quickly and confidently. When we see "not only
to", we insist on "but also
to". Eliminate (CDE).
Between (A) and (B), "amount" cannot be used to modify countable nouns (such as "students"). So (A) is out. (B) must be the right answer.
It is almost always a good idea to read the complete sentence formed by (B) before committing the answer. There, I realized "...but also to the admission of a greater number from underrepresented minorities" does not make sense. A senseless sentence cannot be the right answer. Now, repeat the full process including actually checking answer (CDE) for some insidious tricks that might have eluded me. Finally, I realized the underlined part should end at "a larger amount" rather than "a larger amount of students".