A is simply, flatout wrong. It could work only as part of a longer sentence: "It was a question that I found difficult to answer."
B is perfectly correct.
C is good if the context is right: "The question was so vague that I found it difficult to answer."
D is wrong.
It's a good idea to check with us if an exam answer or a textbook item doesn't make sense to you. It seems they are often wrong.
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