Question

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

Answer 1

11,,21,51 are all  prime numbers

Answer 2

A prime number has exactly 2 different factors, 1 and itself.

11 is divisible only by 1 and 11.
11 is prime.

21 and 51 have a least three factors. They are not prime.
1 has only 1 factor. 1 is not prime.