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 it can only be divided evenly by the number 1 and the number itself, resulting in whole numbers.

In this case, 11 is a prime number because its only divisors are 1 and 11. It cannot be divided evenly by any other number.

The other numbers do not meet the criteria for being prime. For instance, 4 can be divided evenly by 1, 2, and 4; 6 can be divided by 1, 2, 3, and 6; and 9 can be divided by 1, 3, and 9. Each of these has additional divisors besides 1 and itself, which disqualifies them from being prime numbers. Thus, 11 stands out as the only prime number in the list provided.