Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.
GEC, BZX, WUS, RPN, ?

Given: GEC, BZX, WUS, RPN, ?

Logic: Each and every letter is decreasing by-5.

First Letters: G → B → W → R → ?

G -5 → B, B -5→ W, W -5 → R, R -5→ M.

Second Letters: E → Z → U → P → ?

E -5→ Z, Z -5 → U, U -5→ P, P -5 → K.

Third Letters: C → X → S → N → ?

C -5 → X, X -5→ S, S -5 → N, N -5→ I.

First letter: R -5 → M.

Second letter: P -5 → K.

Third letter: N -5 → I.

Thus, the missing term is MKI.

Thus, the correct option is (a) MKI.