Select the letter-cluster from among the given options that can replace the question mark (?) in the following series. CDE, FGH, IJK, ?

Correct — D.
Rule: the alphabet in blocks of three, each block starting 3 letters after the last.

Starts: C(3), F(6), I(9) → next start L(12).
Each cluster is three letters in a row: C D E, F G H, I J K.
So the next cluster is L M N → option (d).

Why the others are wrong

• (a)
  OJB — OJB starts at O, three letters too far: the next block begins at L. Its letters also jump about, O, J, B, where every given cluster runs in a row.
• (b)
  TYC — TYC starts at T and its letters are not consecutive. Every given cluster is three letters in a row, and the row after I J K is L M N.
• (c)
  LJN — LJN has the right first letter, L, but then steps back to J. After L the block must continue with M and N.

A letter-cluster series is several number series read column by column. Convert letters to positions (A = 1 … Z = 26) and take the step in each column.

Here every column climbs by 3: first letters C, F, I, L, second letters D, G, J, M, third letters E, H, K, N. Each cluster is three consecutive letters, so the series is simply the alphabet cut into threes.

Key facts

• C = 3, F = 6, I = 9, L = 12: the first letters climb by 3.
• Blocks of three consecutive letters that start 3 apart cover the alphabet with no gap and no overlap.
• L M N follows I J K with no letter skipped.

Common traps

• Matching only the first letter and choosing LJN

No directly related past PYQ was found.