Three of the following letter cluster pairs are alike in a certain way and thus form a group. Which is the letter cluster pair that does NOT belong to that group?

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

Let's check the options -

First option: WN - RU; W - 9 = N, N + 4 = R; R + 3 = U

Third option: TK - OR; T - 9 = K; K + 4 = O; O + 3 = R

Fourth option: QH - LO; Q - 9 = H; H + 4 = L; L + 3 = O

So, only the letters in the second option (b) follow a different pattern.

Thus, the correct answer is (b).