Arabian Nights are(a)/ a collection of (b)/ very interesting episodes of adventure.(c) No error (d)

In the following questions, some of the sentences have errors and some are correct. Find out which part of a sentence has an error. The number of that part is your answer. If a sentence is free from errors, then your answer is (4) i.e., No error. "The Arabian Nights" (1) / are indeed (2) / an Interesting book. (3) / No error (4)