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.

HEAVY : ICBTZ :: LIGHT : ?

MGHFU

1   2    3    4   5    6   7     8    9  10  11  12  13

 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

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THE LOGIC FOLLOWED HERE IS:

H → (+1) → I

E →  (-2) → C

A → (+1) → B

V →  (-2) → T

Y → (+1) → Z

Similarly,

L → (+1) → M

I →  (-2) → G

G → (+1) → H

H →  (-2) → F

T → (+1) → U

 Thus, MGHFU is correct.