ChrisLele
This question is testing the redundancy of, 'the reason...is/was because', which, to many, sounds correct (mainly because many speak this way). 'The reason' already implies a 'because'. And, for that matter, 'because' gives us a reason, so there is no need to put 'reason' before because. It is this latter case that pertains to the sentence below, because the word 'because' is in the non-underlined part. Therefore, we want to remove 'the reason' from the beginning.
Doing so allows us to get rid of answer (A). (D) and (E) both use a passive construction, which the GMAT typically avoids in favor of the active voice. Therefore (C) is the answer.
The reason the factory did not meet its deadline is because a snowstorm in the northeast delayed delivery of the necessary components by more than a week.
A. The reason the factory did not meet its deadline is
B. The factory was not able to be meeting the deadline
C. The factory did not meet its deadline
D. The deadline of the factory was not met
E. The deadline, of the factory, was not met
I had taken D for the answer, preferring it over C.
Thanks to all of you for helping me to understand the problem better and be careful.