Which of the following is an example of 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, a prime number can only be divided evenly (without a remainder) by 1 and the number itself.

In the case of the number 11, it meets the criteria for being a prime number because its only divisors are 1 and 11. There are no other numbers that can divide 11 evenly, confirming its status as prime.

On the other hand, the other numbers provided do not meet the criteria for prime numbers. The number 4 can be divided by 1, 2, and 4, as can 9 (which can be divided by 1, 3, and 9). The number 15 can be divided by 1, 3, 5, and 15. Therefore, they all have divisors in addition to 1 and themselves, which disqualifies them from being classified as prime.

Thus, the correct choice is the number 11, as it is the only option that fits the definition of a prime number.