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. When considering the choices presented, 11 meets this criterion, as its only divisors are 1 and 11.

In contrast, the other numbers have multiple divisors: 8 can be divided by 1, 2, 4, and 8, making it composite; 9 can be divided by 1, 3, and 9, so it is also composite; and 15 can be divided by 1, 3, 5, and 15, identifying it as composite as well. The uniqueness of 11, having exactly two distinct positive divisors, confirms it as a prime number. Thus, among the choices, 11 stands out as the only prime number.