12. PLAYING WITH NUMBERS
Explanation:
19
We can form 19 pairs of co primes from the 2, 3, 4, 5, 6, 7, 8, 9 which are given below,
(2, 3), (2, 5), (2, 7), (2, 9), (3, 4),(3, 5), (3, 7), (3, 8), (4, 5), (4, 7), (4, 9), (5, 6), (5, 7), (5, 8), (5, 9), (6, 7), (7, 8), (7, 9) and (8, 9)
Hence, the correct answer is option (a).