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.)

The correct option is SPU.

To determine the letter-cluster that does not belong to the group, let us analyze the pattern using the positional values of letters in the English alphabet (where A = 1, B = 2, ..., Z = 26):

1. PMS:
Positional values: P = 16, M = 13, S = 19
$16-3=13$ (P - 3 = M)
$13+6=19$ (M + 6 = S)
Pattern: -3, +6

2. URX:
Positional values: U = 21, R = 18, X = 24
$21-3=18$ (U - 3 = R)
$18+6=24$ (R + 6 = X)
Pattern: -3, +6

3. JGM:
Positional values: J = 10, G = 7, M = 13
$10-3=7$ (J - 3 = G)
$7+6=13$ (G + 6 = M)
Pattern: -3, +6

4. SPU:
Positional values: S = 19, P = 16, U = 21
$19-3=16$ (S - 3 = P)
$16+5=21$ (P + 5 = U)
Pattern: -3, +5

All letter-clusters follow a pattern of subtracting 3 to get the second letter and adding 6 to get the third letter (-3, +6), except for SPU, which adds 5 instead of 6 to get the third letter.

Therefore, SPU does not belong to the group.