What comes next? MOA, NPB, OQC, PRD, ?

Correct — D.
Rule: every letter moves +1 from one cluster to the next.

First letters: M, N, O, P → Q
Second letters: O, P, Q, R → S
Third letters: A, B, C, D → E

The next cluster is QSE → option (d).

Why the others are wrong

• (a)
  QRE — QRE keeps R in the middle, repeating PRD. The middle letters run O, P, Q, R, so the next one is S.
• (b)
  QTE — QTE jumps the middle letter from R to T, a move of +2. Every position here moves +1, which gives S.
• (c)
  RST — RST starts with R, two steps past P, and ends with T instead of E. The first letters need Q, and the third letters continue A, B, C, D to E.

A letter-cluster series is really three series side by side, one per position. Read each position down the list and find its step on its own.

Here all three step +1: M N O P, O P Q R and A B C D. So each cluster is the previous one with every letter pushed forward once, and PRD becomes QSE.

Key facts

• M = 13, P = 16, Q = 17, R = 18, S = 19, D = 4, E = 5.
• Each position of a letter cluster can be read as its own series.
• Here the first and second letters of every cluster are two apart.

Common traps

• Reading the middle letter as fixed because R appears in PRD, which gives QRE
• Checking only the first letter: QRE, QTE and QSE all start with Q

No directly related past PYQ was found.