Which of the following is NOT 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 essence, a prime number can only be divided evenly by 1 and the number itself without leaving a remainder.

When analyzing the options, 11, 17, and 19 are all prime numbers. Each of these numbers can only be divided evenly by 1 and the number itself:

• 11 can be divided by 1 and 11.

• 17 can be divided by 1 and 17.

• 19 can be divided by 1 and 19.

In contrast, 15 is not a prime number because it has additional divisors. Specifically, 15 can be divided evenly by 1, 3, 5, and 15. The presence of these divisors besides 1 and itself means that 15 fails the prime number criteria. Thus, it is categorized as a composite number, which is the accurate distinction for this question.