Select the letter-cluster from among the given options that can replace the question mark (?) in the following series. RST, UVW, XYZ, ?

Correct — C.
Rule: three consecutive letters, each cluster starting 3 after the last.

Starts: R(18), U(21), X(24) → next start 24 + 3 = 27.
Past Z(26), position 27 wraps to A(1).

Three consecutive letters from A: ABC → option (c).

Why the others are wrong

• (a)
  ASD — ASD starts on A correctly but its letters are not consecutive. RST, UVW and XYZ each run three letters in a row, so after A come B and C.
• (b)
  ABD — ABD skips C. Each given cluster is three letters in a row, so after A and B the third letter must be C.
• (d)
  BGA — BGA starts on B, one past the wrap point: X + 3 lands on A. Its letters also jump from B to G and fall back to A.

RST, UVW, XYZ is simply the alphabet cut into blocks of three, starting from R. When a series runs past Z it continues from A, as if the alphabet were a circle, so the block after XYZ is ABC.

In numbers: the starts climb 18, 21, 24, and 24 + 3 = 27. Subtract 26 to bring 27 back to 1, which is A.

Key facts

• R, U, X are positions 18, 21, 24: each start is 3 after the one before.
• Counting past Z restarts at A, so position 27 is A (27 − 26 = 1).
• Three-letter blocks from R end exactly at Z, so the next block begins at A.

Common traps

• Looking for a 27th letter instead of wrapping back to A
• Stopping at the first letter, which ASD and ABD share with the answer

No directly related past PYQ was found.