Which of the following is a prime number?

A prime number is defined as a natural number greater than 1 that has no positive divisors other than 1 and itself. In evaluating the options provided:

The number 19 is a prime number because its only divisors are 1 and 19. It cannot be divided evenly by any other whole numbers, which is the hallmark of prime numbers.

In contrast, the other options can be divided by numbers other than 1 and themselves. For instance, 9 can be divided by 1, 3, and 9; 15 can be divided by 1, 3, 5, and 15; while 21 can be divided by 1, 3, 7, and 21. This capability to be divided by additional numbers disqualifies them from being classified as prime.

Thus, 19 stands out as the only prime number among the given choices.