Select the letter-cluster from among the given options that can replace the question mark (?) in the following series: S C E G, S F K P, S I Q Y, S L W H, ?

Correct — A.
Rule: 1st letter fixed at S; then +3, +6 and +9 in positions 2, 3 and 4, wrapping past Z.

2nd: C(3) → F(6) → I(9) → L(12) → O(15)
3rd: E(5) → K(11) → Q(17) → W(23) → 29 − 26 = C(3)
4th: G(7) → P(16) → Y(25) → 34 − 26 = H(8) → Q(17)

Next cluster: S O C Q → option (a).

Why the others are wrong

• (b)
  S O B Q — B in third place is one letter short. W is 23 and 23 + 6 = 29; subtracting 26 gives 3, which is C, not B.
• (c)
  S P C R — P breaks the +3 run of second letters (C, F, I, L), which gives O. The last letter R misses too: H(8) + 9 = 17 is Q.
• (d)
  S P D R — S P D R is one letter too far in each of the last three places. The rule gives O, C and Q, and this option has P, D and R.

A letter that never changes is a signal that each position runs its own series. Set aside the fixed S and follow the remaining three positions down the terms separately.

The steps here grow in threes, +3, +6, +9, and the last two positions wrap past Z. Subtract 26 from any sum above 26: 29 becomes 3 (C) and 34 becomes 8 (H).

Key facts

• Positions used here: C 3, F 6, I 9, L 12, O 15.
• W is 23 and Y is 25, so +6 from W and +9 from Y both wrap past Z.
• A sum above 26 loses 26: 29 gives C and 34 gives H.

Common traps

• Miscounting the wrap after W: 23 + 6 = 29, and 29 − 26 = 3 is C, so B is one short
• Checking only the first two letters, which leaves S O C Q and S O B Q tied

No directly related past PYQ was found.