What comes next? AX, JQ, SJ, ?

Correct — B.
Rule: first letters +9, second letters −7.

First: A(1) → J(10) → S(19) → 19 + 9 = 28, and 28 − 26 = 2 → B
Second: X(24) → Q(17) → J(10) → 10 − 7 = 3 → C

Next pair: BC → option (b).

Why the others are wrong

• (a)
  BA — BA gets the first letter right but ends in A (1), which is J − 9. The second letters fall by 7 each step, and J − 7 is C.
• (c)
  ZC — ZC stops the first letter at Z (26), which is S + 7. The first letters jump by 9, which carries S past Z to B.
• (d)
  YC — YC puts Y (25) after S, a jump of 6. The first-letter step is +9 (A → J → S), so the next letter wraps past Z to B.

In a letter-pair series, track each position on its own. Turn letters into numbers (A = 1 … Z = 26), find the step for the first letters and the step for the second letters, and continue each separately.

When a step carries a letter past Z, subtract 26 to wrap back to the start: 19 + 9 = 28 → 2 = B. When it goes below A, add 26.

Key facts

• Anchor positions: E5, J10, O15, T20, Y25.
• Wrap past Z by subtracting 26: S(19) + 9 = 28 → B(2).
• Each position in a letter-pair series can follow its own step, here +9 and −7.

Common traps

• Spotting that J and Q are opposite letters (10 + 17 = 27) and hunting for opposite pairs: AX and SJ are not opposites
• Stopping at Z instead of wrapping: S + 9 runs past Z and lands on B

No directly related past PYQ was found.