Select the letter-cluster from among the given options that can replace the question mark (?) in the following series. PQR, STU, VWX, ?

Correct — C.
Rule: the alphabet in order, three letters per term, so each position moves +3.

PQR, STU, VWX run P to X with no gap.
The next three letters are Y, Z and then A, because the count restarts after Z.

That gives YZA → option (c).

Why the others are wrong

• (a)
  YZD — YZD jumps from Z to D, skipping A, B and C. The letter after Z is A, so the third place must be A.
• (b)
  YVA — YVA puts V in the middle. The middle letters run Q, T, W, so the next is W + 3 = Z, and V would be a step back from W.
• (d)
  DFR — DFR fits none of the three positions. The first letters run P, S, V, so the next must be V + 3 = Y.

This letter-cluster series writes the alphabet out in groups of three. PQR, STU, VWX are nine letters in a row, so each position climbs by 3 from one term to the next: P, S, V, then Y.

The care point is the end of the alphabet. Letter series treat it as a loop, so the letter after Z is A.

Key facts

• Y = 25 and Z = 26, and one more step wraps to 1, which is A.
• Three consecutive letters per term means every position moves +3: P, S, V, Y.
• The middle letters run Q, T, W, Z and the last letters R, U, X, A.

Common traps

• Assuming the series cannot pass Z and picking a cluster that skips to D
• Checking only the first letter, which three options share

No directly related past PYQ was found.