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

in all other groups, third and fourth letters consecutive alphabets.

In all other groups, there is a gap of one letter between first and second letters as they appear in English alphabet.

Except '4' , all three are same because in those three options first and forth; and second and third are opposite letters.

Except 'HJMPT' , all three are same and having deference in increasing order.

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

The pattern used for classification is placement of alphabet in the order (-3, -3 -3). Since, option (b) does not follow the pattern, it is odd in the group.

3216 ? 3 + 2 + 6 - 1 ? 10
2338 ? 2 + 3 + 8 - 3 ? 10
3235 ? 3 + 2 + 5 - 0 ? 10
2015 ? 2 + 0 + 5 - 1 ? 6
Therefore, 2015 is different from the other three.

Only this is not an instrument action pair.

Frogs don't bleat, they croak.