Answer: A) Leave as is.
Step-by-step explanation: "But" can join two independent clauses (clauses that make sense by themselves) or an independent clause with a dependent clause (a clause that doesn't make sense on its own). When "but" is joining two independent clauses, we need to put a comma before the "but." If it is joining a independent clause with a dependent clause, we don't have to put any punctuation before "but." In the given sentence we have two independent clauses "Harvey finished typing his research paper" and "he needed to proofread it before submitting it to the professor" so the correct answer is leave as is (with a comma).