Which one is a prime number?,11,21,51,

11). A prime number is a natural number greater than 1 that cannot be formed by multiplying two smaller natural numbers.

11 is only divisible by 1 and 11, it is a prime number. 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.