it's "D"
B implies that she does either of the two, but the original sentence says she does both.
C implies that being quick on completing her homework makes her do her work correctly
A would make a bit of sense, but it's wrong with the "but" in the sentence as it implies that she shouldn't get her work done fast even though she gets her work done correctly.
D makes most sense as it refers to her doing good at her work which would help her finish her homrwork.
(Sry if the explaining is a bit bad, I'm not good with forming my thoughts)