The correct option is B (b)
Let’s examine the sentence: “(a) The pile of books (b) are (c) missing.” The subject of the helping verb ‘are’ is singular, i.e., “pile.” Hence, helping verb for singular noun would be used in the sentence. So, ‘are’ would be replaced by ‘is.’ Hence, the error is in Part B of the sentence. So, Option B is the correct answer.