Which of the following is an example of 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. This means that a prime number cannot be formed by multiplying two smaller natural numbers.

In this case, 11 is a prime number because its only divisors are 1 and 11. It cannot be divided evenly by any other number. This property makes it fundamental in various areas of mathematics, particularly in number theory.

The other numbers mentioned do not fit the definition of a prime number. For instance, 4 is divisible by 1, 2, and 4, meaning it has divisors other than 1 and itself. Similarly, 9 can be divided by 1, 3, and 9, and therefore also has factors beyond 1 and itself, making it composite. Lastly, 15 can be divided by 1, 3, 5, and 15, which confirms that it is also not prime since it has multiple divisors. This understanding emphasizes the uniqueness of prime numbers in arithmetic operations and their significance in various mathematical applications.