What comes next: ZY, XW, VU, TS, ?

Correct — A.
Rule: count backwards from Z, two letters to a term.

Z(26) Y(25), X(24) W(23), V(22) U(21), T(20) S(19): one unbroken countdown.
The next two letters are R(18) and Q(17).
So the fifth term is RQ → option (a).

Why the others are wrong

• (b)
  PQ — PQ runs forwards (P then Q) and leaves out R. Every given term is two adjacent letters in falling order, like T then S.
• (c)
  PR — PR runs forwards with Q missing between P and R. The countdown after S is R then Q, with no gap and no reversal.
• (d)
  QP — QP falls in the right direction but starts at Q, skipping R. The letter straight after S, counting backwards, is R, so the term must begin with R.

This letter series is a plain walk through the alphabet. Write the position numbers under the letters: Z = 26, Y = 25, and so on down to A = 1.

Here the positions read 26 25, 24 23, 22 21, 20 19: a single countdown cut into pairs, with nothing skipped and nothing repeated.

Once the countdown is visible, the next term is just the next two numbers, 18 and 17: R and Q.

Key facts

• Positions at the end of the alphabet: Z 26, Y 25, X 24, W 23, V 22, U 21, T 20, S 19, R 18, Q 17.
• In ZY, XW, VU, TS the first letters fall by 2 each time (26, 24, 22, 20), so the next term starts at 18.

Common traps

• Choosing QP because it falls in the right direction, without checking that it starts from R.
• Reading only the first letters (Z, X, V, T) and guessing the second letter instead of continuing the countdown.

No directly related past PYQ was found.