Correct option is D
Given: CF, IL, OR,?
First Letters: C, I, O, ?
Each letter increases by 6.
· C (3) + 6 = I (9)
· I (9) + 6 = O (15)
· (15) + 6 = U (21)
Second Letters: F, L, R, ?
Each letter also increases by 6.
· F (6) + 6 = L (12)
· L (12) + 6 = R (18)
· R (18) + 6 = X (24)
So, the missing cluster is UX.
Correct answer is (d) UX.