Identify the composite number

Question

61
71
51
31

Answer 1

61 is not a composite number because it only has two factors, 1 and 61.
71 is not a composite number because it only has two factors, 1 and 71.
51 is a composite number because it has factors other than 1 and itself. Its factors are 1, 3, 17, and 51.
31 is not a composite number because it only has two factors, 1 and 31.