To identify which of the given numbers is prime, we must recall the definition of prime numbers. A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself.
Let's evaluate each of the given numbers in turn:
A. 21 - The number 21 is divisible by 1, 3, 7, and 21. Because it has divisors other than 1 and itself, it is not a prime number.
B. 14 - The number 14 is divisible by 1, 2, 7, and 14. As it is divisible by numbers other than 1 and itself, 14 is not a prime number.
C. 13 - The number 13 is divisible by only 1 and 13. There are no other numbers that divide 13 evenly, which makes it a prime number.
D. 33 - The number 33 is divisible by 1, 3, 11, and 33. Since it has divisors other than 1 and itself, it is not a prime number.
Therefore, the correct answer is:
C. 13
