For this sentence make sure to note how the sentence requires a conjunction. As is, (answer A), we have a comma splice.
The answers (A), (B), and (E) make the underlined part an adverbial clause. Therefore, only (C) and (D) work because they use the subordinating conjunction 'but', thereby creating full sentences.
(D) is an example of verbiage, so that leaves answer (C).
Hope that helps
Magoosh Test Prep