What comes next? RST, STU, TUV, UVW, VWX, WXY, XYZ, YZA, ?

Correct — A.
Rule: move every letter +1 each step, wrapping Z round to A.

WXY → XYZ
XYZ → YZA (Z + 1 = A)
YZA → Y+1, Z+1, A+1 = ZAB

The first letters alone run R, S, T … X, Y, so the ninth term starts with Z → option (a).

Why the others are wrong

• (b)
  RST — RST repeats the first term, as if the series restarts after Z. It does not: XYZ → YZA shows the letters wrap round and carry on, so YZA becomes ZAB.
• (c)
  ABC — ABC skips a term. It starts two letters after Y, but each term starts only one letter later than the one before. ABC is the term after ZAB.
• (d)
  ACD — ACD skips B, so it is not three consecutive letters. Every term here is a run of three consecutive letters, and ZAB (Z, A, B) keeps that.

Treat the alphabet as a circle: after Z comes A again. The series shows this itself, since XYZ is followed by YZA.

Each term is three consecutive letters, and the whole block slides forward one place per step. Tracking the first letter is enough: R, S, T, U, V, W, X, Y, so the next term starts with Z.

Key facts

• After Z the count restarts at A: subtract 26 whenever a position passes 26.
• In a sliding series of consecutive letters, the first letters alone form the series.
• Positions: R 18, S 19, T 20, X 24, Y 25, Z 26.

Common traps

• Stopping at Z and assuming the series must restart at its first term (RST)
• Jumping from YZA straight to ABC and skipping the ZAB term the wrap produces

No directly related past PYQ was found.