Which number 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 5 meets this criterion because it can only be divided evenly by 1 and 5. This means it has exactly two distinct positive divisors.

To understand further, let's consider the other numbers. One is not considered a prime number since it doesn’t meet the condition of having two distinct divisors. The number four is not prime because it has three divisors: 1, 2, and 4. Similarly, eight has four divisors: 1, 2, 4, and 8. These characteristics of having more than two positive divisors disqualify both four and eight from being prime. Thus, among the provided choices, 5 is the only prime number.