Based on the English alphabetical order, three of the following letter cluster pairs are alike in a certain way and thus form a group. Which letter cluster pair DOES NOT belong to that group? (Note: The odd one out is not based on the number of consonants/vowels or their position in the letter cluster.)​​

Let's check the options:

Option (a): NJ - SO $$\rightarrow$$​ N + 5 = S; J + 5 = O

Option (b): GB - KF $$\rightarrow$$​ G + 4 = K; B + 4 = F

Option (c): MH - QL $$\rightarrow$$​ M + 4 = Q; H + 4 = L

Option (d): VQ - ZU $$\rightarrow$$​ V + 4 = Z; Q + 4 = U

So, only the first option is different from the other options.

Thus, the correct answer is (a).