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 determine which letter-cluster does not belong to the group, we analyze the alphabetical positions of the letters in each cluster. Let us write down the positional values of the letters in the English alphabet:
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, we inspect the relationships between the positions of the letters in each option:

1. For RXU:
R = 18, X = 24, U = 21
Pattern:
$R+6=X$ (since $18+6=24$)
$X-3=U$ (since $24-3=21$)

2. For IOL:
I = 9, O = 15, L = 12
Pattern:
$I+6=O$ (since $9+6=15$)
$O-3=L$ (since $15-3=12$)

3. For FLI:
F = 6, L = 12, I = 9
Pattern:
$F+6=L$ (since $6+6=12$)
$L-3=I$ (since $12-3=9$)

4. For MSO:
M = 13, S = 19, O = 15
Pattern:
$M+6=S$ (since $13+6=19$)
$S-4=O$ (since $19-4=15$)

As shown above, the letter-clusters RXU, IOL, and FLI follow a consistent pattern where the second letter is obtained by adding 6 to the position of the first, and the third letter is obtained by subtracting 3 from the position of the second. MSO does not follow this group's pattern, as it subtracts 4 instead of 3.

Therefore, the odd letter-cluster is MSO.