Three of the following four letter-cluster pairs are alike in a certain way and thus form a group. Which is the letter-cluster pair that does NOT belong to that group? (Note: The odd one out is not based on the number of consonants/vowels or their positions in the letter-cluster.)

Logic: 1st letter increasing by +7 and 2nd letter decreasing by -6

Let's check all the options: 

Option (a): CG → JB

C + 7 = J, G - 6 = A $$\neq$$ B (not follow)​

Option (b): LP → SJ

L + 7 = S, P - 6 = I 

Option (c): IM → PG

I + 7 = P, M - 6 = F

Option (d): FJ → MD

F + 7 = M, J - 6 = C

Option B, C and D follow the logic of (+7, -6) except option A.

Thus, the correct Option (a): CG → JB​