This was not a fun flaw question.
At its absolute most basic, the stimulus says:
Conclusion: Not X.
Premise: If we BELIEVED X -> Y.
The assumption here is subtle: Since not Y, not X.
But why on Earth would we make that jump based on the single premise we are given? Also, notice we are drawing a conclusion based on what would be true if we BELIEVED otherwise, not if the case it WERE otherwise.
Answers:
A. A true belief (X) can have bad consequences (maybe, not Y). In other words, the author is failing to consider the possibility that X -> not Y. If this is true, the argument doesn´t work.
B. The author establishes one claim to not be true, but where is the other???
C. Irrelevant, there are no motives mentioned.
D. No implication that the most negative outcome must occur.
E. There is no group of individuals being compared to another.
This is also how I thought about the question on the test. My first time reading the answers, none stood out. I went back and visited A and thought it was suggesting that maybe if we´d told the students that HD was the only alternative, they would have answered differently. The flaw is assuming that if they´d known, they still would vote to replace the current dining service with HD. But we have no reason to think that is true.