All of these answers are correct, depending on the tense of the sentence. If this is a narrator, C.) is your best choice. If this is a dialogue for a character, B.) fits best. Overall, judging from sentence structure, C.) is your best answer.
Carefully balanced on the arm of the chair was a stack of books. The answer is (was) because it said "a stack of books". You don't look at 'books'; but you look at a stack. Collective nouns (like 'stack') match with singular verbs. Try (were). Doesn't it sound weird? (Are) doesn't fit either.
