The Christmas holiday brought a brief respite to opposing

Hi 2013gmat.

First of all, meaning is key for this question. The intended meaning is: The Christmas holiday brought a brief respite because an unofficial ceasefire spread along the Western.

We have: as, in that, for, because, since can be use interchangeably. But GMAT prefers precision.

The Christmas holiday brought a brief respite to opposing armies during World War I in that an unofficial ceasefire spread along the Western Front on Christmas Eve, 1914.

A. armies during World War I in that an unofficial ceasefire spread along the Western Front on Christmas Eve, 1914
Wrong. "in that" = "because", however GMAT considers "in that" over-formal. Moreover, for the same meaning, GMAT prefers more precise usage (because >>> in that)

B. armies during World War I, spreading an unofficial ceasefire along the Western Front on Christmas Eve, 1914
Wrong. Modifier problem. "spreading" is Verb-ing modifier + comma ==> modifies a preceding clause. The respite did not lead to an unofficial ceasefire. Wrong meaning.

C. armies during World War I when they spread an unofficial ceasefire along the Western Front on Christmas Eve, 1914
Wrong. Pronoun problem. "they" refers to whom?

D. armies during World War I, for an unofficial ceasefire spread along the Western Front on Christmas Eve, 1914
Correct. "for" = "because". So D keeps the intended meaning.

E. armies during World War I by the spread of an unofficial ceasefire along the Western Front on Christmas Eve, 1914
Wrong. Too wordy. In addition, GMAT prefers the order V-A-N (Verb-Adjective-Noun). If you can use verb, why you use a noun? Hence, E is incorrect.

Hope it helps.