Which of the following numbers is not a prime number?

Let's test the given options to see if they have any factors other than 1 and themselves.For 731:We test divisibility by prime numbers up to $\sqrt{731}\approx 27$.Primes to check: 2, 3, 5, 7, 11, 13, 17, 19, 23.
$$731÷17=43$$
Since 731 is divisible by 17 ($17\times 43=731$), it is a composite number and not a prime number.For 227:$\sqrt{227}\approx 15$. Checking primes up to 13 (2, 3, 5, 7, 11, 13) shows none divide 227. It is a prime number.For 347:$\sqrt{347}\approx 18.6$. Checking primes up to 17 (2, 3, 5, 7, 11, 13, 17) shows none divide 347. It is a prime number.For 461:$\sqrt{461}\approx 21.4$. Checking primes up to 19 (2, 3, 5, 7, 11, 13, 17, 19) shows none divide 461. It is a prime number.Therefore, 731 is not a prime number.