Question

Which verb best completes the sentence?

Carefully balanced on the arm of the chair __________ a stack of books.

A.
was

B.
are

C.
were

Answer 1

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.

Answer 2

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.