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 letter-cluster is not based on the number of consonants/vowels or their position in the letter-cluster.)

To find the odd letter-cluster, let us analyze the positions of the letters in each cluster according to the English alphabetical order (where A = 1, B = 2, ..., Z = 26):

1. TQV:
T = 20, Q = 17, V = 22
Difference between 1st and 2nd letter: 17 - 20 = -3
Difference between 2nd and 3rd letter: 22 - 17 = +5
Pattern: (-3, +5)

2. NKP:
N = 14, K = 11, P = 16
Difference between 1st and 2nd letter: 11 - 14 = -3
Difference between 2nd and 3rd letter: 16 - 11 = +5
Pattern: (-3, +5)

3. KHM:
K = 11, H = 8, M = 13
Difference between 1st and 2nd letter: 8 - 11 = -3
Difference between 2nd and 3rd letter: 13 - 8 = +5
Pattern: (-3, +5)

4. QNR:
Q = 17, N = 14, R = 18
Difference between 1st and 2nd letter: 14 - 17 = -3
Difference between 2nd and 3rd letter: 18 - 14 = +4
Pattern: (-3, +4)

Comparing the patterns, we can see that TQV, NKP, and KHM all follow the pattern of subtracting 3 from the first letter and adding 5 to the second letter. However, QNR follows a different pattern of subtracting 3 and adding 4. Therefore, QNR is the odd letter-cluster that does not belong to the group.