Select the letter-cluster from among the given options that can replace the question mark (?) in the following series: C D E F, C K S A, C R G V, C Y U Q, ?

Correct — D.
Rule: C stays fixed, and positions 2, 3, 4 move +7, +14, +21.

Second: D, K, R, Y → Y(25) + 7 = 32 − 26 = F(6).
Third: E, S, G, U → U(21) + 14 = 35 − 26 = I(9).
Fourth: F, A, V, Q → Q(17) + 21 = 38 − 26 = L(12).

That gives C F I L → option (d).

Why the others are wrong

• (a)
  C E H K — C E H K is one letter short at every moving position. Y + 7 is F, not E, U + 14 is I, not H, and Q + 21 is L, not K.
• (b)
  C F H K — C F H K gets F right, then falls one short at both remaining positions. The third letter must be U + 14 = I, and the fourth Q + 21 = L.
• (c)
  C F H L — C F H L gets F and L right and misses the third letter. The third letters move +14 from U, and 21 + 14 = 35 − 26 = 9, which is I, not H.

The C at the front never changes, so the series lives in positions two, three and four. Read each one down the terms as position numbers.

The steps are multiples of 7: +7, +14, +21. Large steps are easier counted backwards, because on a 26-letter wheel +14 is −12 and +21 is −5.

So U − 12 = I and Q − 5 = L, with no totals to reduce.

Key facts

• Y = 25, U = 21 and Q = 17 become F = 6, I = 9 and L = 12.
• On a 26-letter wheel, +14 is the same move as −12 and +21 the same as −5.
• The steps +7, +14, +21 are one, two and three times 7.

Common traps

• Accepting C F H L because its second and fourth letters check out, when the third must be I
• Counting +14 forward one letter at a time and slipping by one, landing on H

No directly related past PYQ was found.