Answer : 3. "21"
Explanation :

Answer: C) 21 Explanation: A prime number is a whole number greater than 1 whose only factors are 1 and itself.   Factors of 5 are 1, 5 Factors of 11 are 1, 11 Factors of 21 are 1, 3, 7, 21 Factors of 37 are 1, 37.   Hence, according to the definition of a prime number, 21 is not a prime number as it has more than two factors.