12. PLAYING WITH NUMBERS
Explanation:
23
23 has only two factors 1 and 23, Therfore, it is a prime number.
51 has three factors 1, 3 and 17, Therfore, it is a composite number.
38 has three factors 1, 2 and 19, Therfore, it is a composite number.
26 has three factors 1, 2 and 13, Therefore, it is a composite number.
Hence, the correct answer is option (a).