Select the letter-cluster from among the given options that can replace the question mark (?) in the following series. P R T, O Q S, N P R, ?

Correct — D.
Rule: each column falls by 1, read column by column.

First letters: P, O, N → M.
Second letters: R, Q, P → O.
Third letters: T, S, R → Q.

The next cluster is MOQ → option (d).

Why the others are wrong

• (a)
  GAS — GAS starts with G, but the first column falls P, O, N, so the next letter is M. None of its three letters fits its column.
• (b)
  MJS — MJS has the right M, then J where the second column (R, Q, P) needs O, and S where the third column needs Q.
• (c)
  GHA — GHA breaks all three columns. The next letters are M, O and Q, one step back from N, P and R.

A letter-cluster series is three letter series stacked together. Read the first letters of every term, then the second letters, then the third, and find the step in each column.

Here every column moves back one letter: P(16), O(15), N(14), M(13). A second check works inside each cluster: its letters climb by +2, as in P, R, T and M, O, Q.

Key facts

• P = 16, O = 15, N = 14, M = 13: the first letters fall by 1.
• R = 18, Q = 17, P = 16, O = 15: the second letters fall by 1.
• Inside each cluster the letters step +2: M(13), O(15), Q(17).

Common traps

• Checking only the first letter, which leaves MJS and MOQ both standing
• Carrying the +2 step inside a cluster over to the columns, which move by only −1

No directly related past PYQ was found.