Based on the English alphabetical order, three of the following four letter-clusters are alike in a certain way and thus form a group. Which letter-cluster 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.)​

Logic: 1st letter + 2 = 2nd letter and 2nd letter - 6 = 3rd letter

Now, we check each options.

Option (a): PRL (Follow)

P + 2 = R, R - 6 = L

Option (b): IKE (Follow)​

I + 2 = K, K - 6 = E

Option (c): HJD (Follow)​

H + 2 = J, J - 6 = D

Option (d): PLO (Not Follow)​
P + 2 $$\neq$$​ L, L - 6 $$\neq$$​ O

Thus, correct option is (d).