Which number 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 other words, it cannot be formed by multiplying two smaller natural numbers.

In this question, 11 meets the criteria for a prime number. Its only divisors are 1 and 11, meaning it cannot be divided evenly by any other integer.

On the other hand, the other numbers listed do not qualify as prime:

• 9 can be divided evenly by 1, 3, and 9, highlighting that it has multiple factors.
• 15 has divisors of 1, 3, 5, and 15, showing it is also not prime.
• 21 can be divided by 1, 3, 7, and 21, confirming it is not a prime number either.

Thus, 11 is the only number in the list that is prime, as it exclusively meets the definition without any other factors present.