To determine which number is not a prime number, we need to check each option for divisibility other than by 1 and itself. Let's analyze the given options:
- 241: The divisibility test shows that 241 is not divisible by any prime number up to โ241 (which is approximately 15.5), such as 2, 3, 5, 7, 11, and 13. As it is not divisible, 241 is a prime number.
- 337: Again, testing divisibility by all prime numbers up to โ337 (approximately 18.4), like 2, 3, 5, 7, 11, 13, and 17, confirms it is not divisible. Thus, 337 is a prime number.
- 391: Check for divisibility by primes up to โ391 (approximately 19.8). Dividing by 17, we find that 391 รท 17 = 23, indicating that 391 is divisible by 17. Therefore, 391 is not a prime number.
- 571: Check primes up to โ571 (approximately 23.9). It is not divisible by 2, 3, 5, 7, 11, 13, 17, 19, and 23, demonstrating that 571 is also a prime number.
Therefore, among the options given, 391 is not a prime number.