Select the option that is related to the third letter-cluster in the same way as the second letter-cluster is related to the first letter-cluster.

PG : DM :: GO : ?

LD

A   B   C    D   E    F   G    H    I    J    K    L   M

 ↓    ↓    ↓    ↓    ↓     ↓   ↓    ↓     ↓   ↓    ↓    ↓    ↓

 Z   Y    X   W   V    U   T    S    R   Q   P    O   N

26  25  24  23  22  21  20  19  18  17  16  15  14

******************************************

PG : DM :: GO : ?

The  logic followed here is :

REVERSE THE FORMAT

PG ⇔ GP

 G → (-3 ) → D

 P → (-3) → M

SIMILARLY

GO ⇔ OG

 O → (-3 ) → L

 G → (-3) → D