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

ABC6 : DEF15 :: GHI24 : JKL33 :: PQR51 : ?

STU60

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

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

ABC6 : DEF15 :: GHI24 : JKL33 :: PQR51 : ?

The  logic followed here is :

A → (+3) → D

A → (+3) → E

A → (+3) → F

A → (+9) → 15

AND

G → (+3) → J

H → (+3) → K

I → (+3) → L

24 → (+9) → 33

SIMILARLY

P → (+3) → S

Q → (+3) → T

R → (+3) → U

51 → (+9) → 60