Explanation

Let's check each option:

1. 31 = 2 + 29 (both prime)

2. 19 = 2 + 17 (both prime)

3. 23 = No combination works (e.g., 2 + 21, 21 is not prime)

4. 7 = 2 + 5 (both prime)

The answer is 23.