Correct Answer: 21

Solution : Let's check the options –
First option: 11→ The factors of 11 are 1 and 13. So, 11 is a prime number.
Second option: 17→ The factors of 17 are 1 and 17. So, 17 is a prime number.
Third option: 19→ The factors of 19 are 1 and 19. So, 19 is a prime number.
Fourth option: 21→ The factors of 21 are 1, 3, 7 and 21. So, 21 is not a prime number.

Hence, the f ourth option is correct.