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

The correct answer is NT - XT.

To find the odd letter-cluster pair out, let us analyze the pattern of positional values in the English alphabet for each pair.

Let us consider the alphabetical positions of each letter:
A=1, B=2, C=3, D=4, E=5, F=6, G=7, H=8, I=9, J=10, K=11, L=12, M=13, N=14, O=15, P=16, Q=17, R=18, S=19, T=20, U=21, V=22, W=23, X=24, Y=25, Z=26.

Now, let us examine each option by looking at the differences between the positional values of the letters:

1. OT - XS:
First letter: O (15) → X (24): +9
Second letter: T (20) → S (19): -1

2. HM - QL:
First letter: H (8) → Q (17): +9
Second letter: M (13) → L (12): -1

3. AF - JE:
First letter: A (1) → J (10): +9
Second letter: F (6) → E (5): -1

4. NT - XT:
First letter: N (14) → X (24): +10
Second letter: T (20) → T (20): 0

As we can see, OT - XS, HM - QL, and AF - JE all follow the logic where the first letter advances by +9 and the second letter goes back by -1. However, NT - XT does not follow this pattern.

Therefore, NT - XT is the one that does not belong to the group.