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 one out is not based on the number of consonants/vowels or their position in the letter-cluster.)

Logic: 1st letter - 3 = 2nd letter and 2nd letter - 6 = 3rd letter
Now, we check each options.
​Option (a): ​DAU

D - 3 = A, A - 6 = U
​Option (b): ​ROI

R - 3 = O, O - 6 = I
​Option (c): ​TQK

T - 3 = Q, Q - 6 = K
​Option (d): ​FCV

F - 3 = C, C - 6 $$\ne$$ V​
Thus, correct option is (d).