Select the letter-cluster from among the given options that can replace the question mark (?) in the following series: AZ, BY, CX, ?

Correct — A.
Rule: 1st letter +1, 2nd letter −1, so each pair is a letter and its opposite.

1st: A → B → C → D
2nd: Z → Y → X → W

Check: D(4) + W(23) = 27, the same as A(1) + Z(26).
Next term: DW → option (a).

Why the others are wrong

• (b)
  DU — U is X − 3. The second letter steps back by 1 each term, so after X comes W, not U.
• (c)
  EV — EV is the fifth term, one step too far. After CX comes DW, and EV follows that.
• (d)
  DX — DX moves the first letter but leaves the second at X. The second letter must also move, back one place to W.

Opposite-letter pairs run A–Z, B–Y, C–X, D–W and on. The opposite of the letter at position n is the letter at 27 − n, so every pair adds to 27.

Knowing the pairs turns this series into a lookup: the next first letter is D, and D's opposite is W.

Key facts

• The opposite of the letter at position n is the letter at position 27 − n.
• A–Z, B–Y, C–X and D–W are the first four opposite pairs.
• E–V, F–U, G–T, H–S and M–N are opposite pairs too.

Common traps

• Moving only one of the two letters
• Skipping a term and answering the fifth, EV

No directly related past PYQ was found.