From the comments, we find that the actual question in question is
Why did the civil rights movement start in the postwar period?
And what's in question is whether this means
What reasons prompted the postwar civil rights movement?
as the test-taker claims or
What reasons associated with the postwar period prompted the civil
as the test-maker claims.
From a strictly grammatical point of view, the test-taker can make a reasoned argument. The problem is that grammar can take him only so far, and that doesn't extend to the semantics of the question.
Presumably, we're talking about the US civil rights movement in the period after World War II. Also, presumably, the test-taker has studied that period and the preceding period that we may call the Golden Age of American Lynching (roughly 1872-1942). If the test-taker had been paying attention, he should have been able to figure out that if oppression were sufficient to explain the movement, then that movement would surely have started during the GAofAL. The interesting answer therefore must take into account why oppression could help trigger a civil rights movement after the Second World War when it couldn't do so after the American Civil War.*
This is a semantic issue about the meaning of events in American history, and the instructor is within his or her rights to expect that students will grasp that much. Which is why an appeal to syntax will be met with stony disapproval.
*actually after Reconstruction.