Select the letter-pair that can replace the question mark (?) in the following series.
MR, NU, OX, PA, ?

In the given series, MR, NU, OX, PA

First position letters of each letter-pair i.e.., M,N,O,P are in a sequence as per alphabetical order. Therefore, next letter would be Q

Second position letters of each letter-pair i.e.., R,U,X,A are having two letters between each of them. Therefore, next possible letter after A will be D with B,C between them.

MR ==> M+1 R+3 ==> NU

NU ==> N+1 U+3 ==> OX

OX ==> O+1 X+3 ==> PA

PA ==> P+1 A+3 ==> QD

Therefore, Correct letter pair in the given choice is QD.