​Select the alphanumeric-cluster that can replace the question mark (?) in the following series?

S-81, Q-64, ? , M-36, K-25

Logic: Letters decrease by -2, and numbers follow square pattern (9², 8², 7², ...)

Pattern in letters (-2 each time)

S → Q → O → M → K

S -2 → Q; Q -2 → O; O -2 → M; M -2 → K

Pattern in numbers (Perfect squares)

81 = 9²

64 = 8²

36 = 6²

25 = 5²

Therefore, the missing term = O-49

Thus, the correct option is (a)