In the following question, four letter pairs are given. The letters on left side of (-) is related to the letters on the right side of (-) with some Logic/Rule/Relation. Three are similar on basis of same Logic/Rule/Relation. Select the odd one out from the given alternatives.

Logic: 1st letter + 6 = 3rd letter and 2nd letter + 6 = 4th letter

Now, we check each options.

Option (a): ET - KZ (Follow)

E + 6 = K, T + 6 = Z

Option (b): NP - TW (Not Follow)​

N + 6 = T, P + 6 $$\neq$$​ W

Option (c): XM - DS (Follow)​

X + 6 = D, M + 6 = S

Option (d): OW - UC (Follow)​

O + 6 = U, W + 6 = C

Thus, correct option is (b).