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. This means that a prime number can only be divided evenly by 1 and the number itself, resulting in a whole number.

In this case, 17 is the prime number among the given options. It can only be divided by 1 and 17 without resulting in a fraction or decimal.

On the other hand, 15 can be divided by 1, 3, 5, and 15, which means it is not prime. Similarly, 21 can be divided by 1, 3, 7, and 21, indicating it is also not a prime number. Lastly, 25 can be divided by 1, 5, and 25, thus making it non-prime as well. Therefore, the only option that meets the criteria for being a prime number is 17.