Correct option is A
Logic: Position of the 1st letter + Position of the 2nd letter = Position of the 3rd letter.

Given Series: CIL, QDU, PFV, KN?
Cluster: CIL
C = 3, I = 9
3 + 9 = 12, which corresponds to L
Cluster: QDU
Q = 17, D = 4
17 + 4 = 21, which corresponds to U
Cluster: PFV
P = 16, F = 6
16 + 6 = 22, which corresponds to V
Cluster: KN?
K = 11, N = 14
11 + 14 = 25, which corresponds to Y
The missing letter is Y.
Correct answer: (a) Y