Which one of the following numbers is NOT a prime number?

Correct option is A

Given:

Options: 231, 313, 211, 241

Concept Used:
A prime number has exactly two distinct positive divisors: 1 and itself.

Solution:

231:
Sum of digits = 2 + 3 + 1 = 6 → divisible by 3
231 is divisible by 3 → Not prime

313 → Prime

211 → Prime

241 → Prime

Thus, option (a) is right.