Which of the following is not a prime number?

Concept: A prime number is a natural number greater than 1 whose only positive divisors are 1 and itself; if any other divisor exists, the number is composite (not prime).

Application: Check each option for a divisor other than 1 and itself, testing primes up to its square root:

1. 43 — primes up to √43 ≈ 6.6 are 2, 3, 5; none divide 43, so 43 is prime.

2. 57 — primes up to √57 ≈ 7.5 are 2, 3, 5, 7; 3 divides 57 since 57 = 3 × 19, so 57 is composite, not prime.

3. 73 — primes up to √73 ≈ 8.5 are 2, 3, 5, 7; none divide 73, so 73 is prime.

4. 101 — primes up to √101 ≈ 10.05 are 2, 3, 5, 7; none divide 101, so 101 is prime.

Cross-check: the digit sum of 57 is 5 + 7 = 12, which is divisible by 3 — confirming that 3 divides 57 and 57 is not prime.

Result: among the given options, only 57 is not a prime number.