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. In this context, 7 fits the criteria perfectly. It can only be divided evenly by 1 and 7, with no other factors involved, confirming its status as prime.

In contrast, the other choices are not prime numbers: 15 can be divided by 1, 3, 5, and 15; 4 can be divided by 1, 2, and 4; and 9 can be divided by 1, 3, and 9. Since each of these numbers has divisors other than 1 and themselves, they do not meet the definition of a prime number. Thus, 7 stands out as the only prime number in the list.