Correct option is D
Given:

LETTER → MGWXJX
L → M (+1)
E → G (+2)
T → W (+3)
T → X (+4)
E → J (+5)
R → X (+6)
Following this pattern, let's decode ICNYSG:
H → I (+1)
A → C (+2)
K → N (+3)
U → Y (+4)
N → S (+5)
A → G (+6)
Thus, HAKUNA is written as ICNYSG in the given code language.
Correct answer is (d) HAKUNA.