Which of the following is not a prime number?

Clearly, 16² > 241.
∴ 16 > √241
Now,
Prime numbers less than 16 are 2, 3, 5, 7, 11, 13.
241 is not divisible by any of them.
∴ 241 is a prime number
In that same fashion,
337 and 571 are also prime numbers.

Now, 391
clearly, 20² > 391
∴ 20 > √391
Now, Prime numbers less than 20 are 2, 3, 5, 7, 11, 13, 17, 19
Here, 391 is divisible by 17.
∴ 391 is not a prime number.