Which of the following is not considered a prime number?

• A. 2
• B. 3
• C. 1
• D. 5

Answer: (C)

A prime number is defined as a natural number greater than 1 that has no positive divisors other than 1 and itself. This means a prime number must be divisible only by 1 and the number itself, resulting in exactly two distinct positive factors.

In this context, the number 1 does not meet the definition of a prime number because it only has one positive divisor, which is itself. Therefore, since 1 does not have two distinct positive factors, it is not classified as a prime number.

On the other hand, the numbers 2, 3, and 5 are all considered prime numbers. Each of these numbers can only be divided without a remainder by 1 and themselves, fulfilling the criteria for prime numbers. Thus, identifying 1 as not being a prime number is accurate and aligned with the fundamental definition of primes in mathematics.