Which of the following is a prime number?

a) 15
b) 21
c) 37
d) 50

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97 are the prime numbers. There are 25 prime numbers between 1 and 100.