Which number 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 other words, it can only be divided evenly by 1 and the number itself.

The number 17 meets the criteria for a prime number. It can only be divided evenly by 1 and 17; there are no other divisors. Therefore, it is confirmed as a prime number.

Other numbers listed, such as 15, 21, and 9, can all be divided by numbers other than 1 and themselves. For instance, 15 can be divided by 3 and 5, 21 can be divided by 3 and 7, and 9 can be divided by 3. Thus, they do not fulfill the definition of a prime number.