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The only answer that makes sense in the blank is "didn't think." The "before"-clause refers to a time period in the past, so the present perfect doesn't work; the present perfect is incompatible with specific past-time adverbials.
Although "wouldn't think" and "don't think" are not impossible, they express a strange meaning that the exercise maker surely did not have in mind. The speaker is not speculating on the way dreams were before he read the story.