Which of the following numbers is a prime number?
(a) 91
(b) 81
(c) 87
(d) 97

Since, factors of
91 = 1 × 7 × 13
81 = 1 × 3 × 3 × 3 × 3
87 = 1 × 3 × 29
97 = 1 × 97
Thus, 81, 87 and 91 all are not prime numbers.
Hence, the correct answer is option (d).