DU FD FS HA IP KY _ _
It's a very poorly written question.
As you've fathomed, the only pairs that are relevant to the answer are 1, 3 & 5, so you are working with:
DU FS IP _ _
To get to the answer KN:
- the first pair is +2, +3 ... +2
- the second pair is -2, -3 ... -2
The trouble is that the question really doesn't give you sufficient information to be certain of that. It could just as easily be +2, +3 ...+4, and -2, -3 ... -4, giving the answer ML.
To give a clear pattern, another two pairs of letters should have been provided.