Which of these numbers is prime?

A prime number is defined as a natural number greater than 1 that has no positive divisors other than 1 and itself. In this case, the number 17 fits this definition perfectly.

To determine if 17 is prime, we look for divisors: the only integers that can divide 17 without leaving a remainder are 1 and 17 itself. There are no other factors, confirming that 17 has exactly two distinct positive divisors.

In contrast, the other numbers listed either have more than two divisors or are not greater than 1. For example, 4 can be divided by 1, 2, and 4; 15 can be divided by 1, 3, 5, and 15; and 21 can be divided by 1, 3, 7, and 21. Because these numbers each have at least one additional divisor, they do not meet the criteria for being prime. Thus, 17 is the only prime number in the list.