Four letter-clusters have been given, out of which three are alike in some manner and one is different. Select the letter-cluster that is different.

Note: The odd one out is not based on the number of consonants/vowels or their position in the letter cluster

To find the letter-cluster that is different from the others, let us analyze the positions of the letters in the English alphabet (where A = 1, B = 2, ..., Z = 26) and check the patterns of differences between consecutive letters in each cluster.

1. PRUZ:
P = 16
R = 18
U = 21
Z = 26
Differences between consecutive letters:
R - P = 18 - 16 = +2
U - R = 21 - 18 = +3
Z - U = 26 - 21 = +5
The pattern of differences is +2, +3, +5.

2. MORW:
M = 13
O = 15
R = 18
W = 23
Differences between consecutive letters:
O - M = 15 - 13 = +2
R - O = 18 - 15 = +3
W - R = 23 - 18 = +5
The pattern of differences is +2, +3, +5.

3. XZCH:
X = 24
Z = 26
C = 3 (or 29 in a circular sequence: 26 + 3 = 29)
H = 8 (or 34 in a circular sequence: 26 + 8 = 34)
Differences between consecutive letters:
Z - X = 26 - 24 = +2
C - Z = 29 - 26 = +3
H - C = 8 - 3 = +5
The pattern of differences is +2, +3, +5.

4. ORTU:
O = 15
R = 18
T = 20
U = 21
Differences between consecutive letters:
R - O = 18 - 15 = +3
T - R = 20 - 18 = +2
U - T = 21 - 20 = +1
The pattern of differences is +3, +2, +1, which does not match the other three clusters.

Therefore, the letter-cluster ORTU is the odd one out.