Which of the following terms will replace the question mark (?) in the given series? OPOR, NOPS, ?, LMRU, KLSV

Correct — C. Convert to alphabet positions and read each of the four slots as its own series.

Rule: slots 1 and 2 fall by one letter each step, slots 3 and 4 rise by one letter each step.

Slot 1: O(15), N(14), M(13), L(12), K(11)
Slot 2: P(16), O(15), N(14), M(13), L(12)
Slot 3: O(15), P(16), Q(17), R(18), S(19)
Slot 4: R(18), S(19), T(20), U(21), V(22)

Reading the third column down gives MNQT — option (c).

Why the others are wrong

• (a)
  MNTQ — MN is right, but T and Q are the wrong way round. Slot 3 has to fall between P and R, and only Q does that.
• (b)
  MNQP — The fourth letter must be T — one after S in NOPS and one before U in LMRU. P is going the wrong way in a column that rises.
• (d)
  NMQT — QT is right, but the opening pair is swapped. Slot 1 runs O, N, M and slot 2 runs P, O, N, so M must come before N.

A letter-cluster series is four independent series printed side by side. Write A = 1 to Z = 26 under every term, then look down the columns rather than across the words.

Once the columns are visible the rule is usually a small constant step. Here the two left columns walk backwards through the alphabet and the two right columns walk forwards, so the clusters spread apart as the series runs.

Checking a candidate is then a one-line job: it must fit all four columns, not just the two you happened to notice.

Key facts

• Write A = 1 through Z = 26 before hunting for a rule.
• Treat each position in the cluster as a separate sequence.
• In this series slots 1 and 2 step back by one while slots 3 and 4 step forward by one.

Common traps

• Trying to read the four letters as a single word instead of four columns.
• Fixing the direction of the step from one pair of terms and not checking it against the rest.

No directly related past PYQ was found.