In all other groups, there is a gap of two letters as in the alphabet between second and third letters.

Except option (d), sum of all other letters given is equal to 27. And given number represents the possition of lst letters .

Except 'PRU', all others having same gap but 'PRU' is not following the same rule.

Except (d), all three are same because in those three options first and fourth, second and third are opposite letters.

In all other groups, the second and first letters are three steps ahead of third and second letters, respectively.

In all other groups, the third and first letters are in alphabetical order

In all other groups, there is a gap of one letters as in the originaly alphabet between second and fourth letters.

In all other groups, there is a gap of one letters as in the alphabet between second and third letters.

In all other groups, there is a gap of one letters as in the alphabet between second first and fourth letters.

Except OLTF, none of the groups of letters has any vowel.