Which letter-cluster will replace the question mark (?) to complete the given series?
RXBG, OVEK, ?, IRKS, FPNW

Correct option is C

Given: RXBG, OVEK, ?, IRKS, FPNW
From RXBG to OVEK
First letter R(18) – 3 = O(15)
Second letter: X(24) – 2 = V(22)
Third letter: B(2) + 3 = E(5)
Forth letter G(7) + 4 = K (11)
Logic: 1st letter – 3, second letter – 2, 3rd letter +3 and 4th letter +4.
OVEK, ?
First letter: O(15) – 3 = L(12)
Second letter: V(22) – 2 = T(20)
Third letter: E(5) + 3 = H(8)
Forth letter: K (11) + 5 O(15)
Correct answer is (c) LTHO.