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

Logic: 1st letter + 1 = 3rd letter and 2nd letter + 1 = 4th letter

Now, we check each options.

Option (a): CG : DH (Follow)

C + 1 = D and G + 1 = H

Option (b): UE : VF (Follow)​

U + 1 = V and E + 1 = F

Option (c): OS : PQ (Not Follow)​

O + 1 = P and S + 1 = T $$\neq$$ Q​

Option (d): LT : MU (Follow)​

L + 1 = M and T + 1 = U

Thus, correct option is (c).