Which of the following is not 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. The numbers 2, 3, and 5 all meet this criterion: they are greater than 1 and cannot be divided evenly by any other numbers aside from 1 and themselves.

On the other hand, the number 1 does not satisfy the definition of a prime number, as it is not greater than 1. In fact, it is classified as a unit, meaning it has only one positive divisor, which is itself. Therefore, while 2, 3, and 5 are all prime numbers, 1 is not considered prime, making it the correct answer to the question.