What comes next? CE, EG, GI, ?

Correct — D.
Rule: both letters move +2 each term, and each pair's second letter opens the next pair.

First letters: C → E → G → I
Second letters: E → G → I → K

The next pair is IK → option (d).

Why the others are wrong

• (a)
  KN — KN moves the first letter +4 (G→K) and puts N three places after K. Every pair so far is a letter followed by the letter two places on.
• (b)
  KM — KM is the term after IK. It keeps the +2 inside the pair, but its first letter skips I, which must follow G.
• (c)
  JL — JL keeps the +2 inside the pair but moves the first letter +3 (G→J). The first letters run C, E, G in steps of 2, so the next is I.

A letter-pair series runs two series side by side. Read the first letters as one series and the second letters as another, then check the gap inside each pair.

Here all three agree: first letters C, E, G (+2), second letters E, G, I (+2), and each pair is a letter plus the letter two on. So the pairs overlap: each pair's second letter opens the next pair.

That overlap is a quick check: the answer must begin with I, the last letter shown.

Key facts

• C = 3, E = 5, G = 7, I = 9, K = 11.
• Each term is a letter followed by the letter two places on.
• Each term begins with the previous term's second letter.

Common traps

• Choosing KM, the term two steps ahead.
• Checking only the gap inside each pair, which JL also passes.

No directly related past PYQ was found.