This question takes some time, since each of the five choices is structured differently. In cases like this, it's a fair bet that some of the wrong answer choices use a very awkward structure, perhaps including passive voice, and that is the case here.

(A) does so: "was the occasion" is superfluous. This choice isn't awful, but if something better comes along, the excess verbiage is enough to eliminate it. (B) is also close, but it subtly changes the meaning. The sentence says that the army made two attempts--this choice says that the army failed twice, suggesting perhaps that the army succeeded a third time.

(D) repeats something like (B)'s error ("twice impossible") and separates "to drill sufficient holes" and "soldiers," making the sentence less elegant. (E) uses passive voice with the structure built around "was" at the beginning of the sentence. (C) isn't the most graceful sentence ever written, but it sidesteps the mistakes made in the other choices, and it is correct.