Based on the English alphabetical order, three of the following four letter-clusters arealike in a certain way and thus form a group. Which letter-cluster DOES NOT belong tothat group? (Note: The odd one out is not based on the number of consonants/vowels or theirposition in the letter-cluster.)

Logic: Letters are not consecutive letters

Now, we check each options.

Option (a): EF (Not Follow)

E and F are consecutive letters.

Option (b): DO (Follow)​

D and O are  not consecutive letters.

Option (c): AD (Follow)​

A and D are not consecutive letters.

Option (d): CA (Follow)​

C and A are not consecutive letters.

Thus, correct option is (a).