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

ADF, GJL, MPR, ?, YBD

​Given: ADF, GJL, MPR, ?, YBD

Logic: Each letter + 6

1st Position (A, G, M, ?, Y):

A + 6 → G

G + 6 → M

M + 6 → S

S + 6 → Y

2nd Position (D, J, P, ?, B):

D + 6 → J

J + 6 → P

P + 6 → V

V + 6 → B 

3rd Position (F, L, R, ?, D):

F + 6 → L

L + 6 → R

R + 6 → X

X + 6 → D 

The missing letter-cluster is SVX.

Thus, correct option is (a).