Explanation

437 = 19 × 23 and 1073 = 29 × 37, so both are composite. For 797, which is below 29² = 841, testing divisibility by every prime up to 28 (3, 5, 7, 11, 13, 17, 19, 23) leaves no divisor, so 797 is prime. Hence only one of the three numbers is prime (A).