Select the letter-pair that can replace the question mark (?) in the following series.
MR, NU, OX, PA, ?
Answer & explanation
Correct answer: option 4

Explanation:
MR, NU, OX, PA, ?
The logic followed here is :
M → (+1) → N → (+1) → O → (+1) → P → (+1) → Q
R → (+3) → X → (+3) → X → (+3) → A → (+3) → D
So, the letter-cluster that replaces the (?) to complete the series is "QD"