Select the letter-cluster from among the given options that can replace the question mark (?) in the following series. C G K, G K O, K O S,O S W, ?

Correct — C.
Rule: +4 at every position, read down the series.

First letters: C, G, K, O → S.
Second letters: G, K, O, S → W.
Third letters: K, O, S, W → W(23) + 4 = 27, past Z to A(1).

That gives SWA → option (c).

Why the others are wrong

• (a)
  SDA — SDA gets the first and last letters right but not the middle. The second letters run G, K, O, S, so the next one must be W, not D.
• (b)
  KNB — KNB starts with K, which would send the first letters backwards. They climb C, G, K, O in fours, so the next first letter is S.
• (d)
  KJH — KJH fails at the first letter: after O comes S, not K. Its letters also run backwards, K, J, H, where every given cluster climbs in fours.

A letter-cluster series is three number series laid side by side. Convert letters to positions (A = 1 … Z = 26) and read one position at a time down the terms.

Here every position climbs by 4, and the letters inside each cluster are also 4 apart. That is why each cluster repeats the last two letters of the one before: CGK, GKO, KOS, OSW. The answer must therefore begin SW.

Key facts

• A = 1 through Z = 26 is the position code these series run on.
• A step that runs past Z continues from A, so W + 4 = A.
• Here the step between terms (+4) equals the gap inside each cluster (+4), so each cluster shares two letters with the next.

Common traps

• Stopping at W + 4 = 27 and looking for a 27th letter instead of wrapping to A
• Choosing SDA because its first and last letters match

No directly related past PYQ was found.