Which of these numbers is a prime number 1, 11, 21, 51?

11 is a prime number.

By defintion a prime number is a number greater than one and the only whole numbers is it can be divided by to provide a whole number quotient are 1 and itself. 1 is not greater than one so it is not a prime number. 11 is only divisible by 11. 21 is visible by 1, 3, 7, and 21; not a prime number. 51 is divisble by 1, 3, 7, 17, and 51; not a prime number.

11 is the only prime number .