What comes next? CG, EI, GK, ?

Correct — C.
Rule: both letters move +2 at each step.

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

The next term is IM → option (c).

Check the gap inside each term: CG, EI, GK and IM are all 4 apart.

Why the others are wrong

• (a)
  KO — KO keeps the gap of 4 inside the pair but moves two steps at once. It is the term after IM, not the next one.
• (b)
  MP — MP starts at M, six places after G instead of two, and its letters are 3 apart where every given term keeps a gap of 4.
• (d)
  NQ — NQ starts at N, seven places after G, and its letters N and Q are 3 apart, not the 4 that CG, EI and GK share.

In a two-letter series, track each position as its own series. Write the positions: C 3, E 5, G 7 for the first letters and G 7, I 9, K 11 for the second. Both rise by 2.

A second check comes free: within each term the letters are 4 apart. The answer must pass both tests, which rules out KO (right gap, wrong step) and the two options with a gap of 3.

Key facts

• Positions: C = 3, E = 5, G = 7, I = 9, K = 11, M = 13.
• Each term's letters are 4 apart: CG, EI, GK, IM.
• Each term starts 2 letters after the one before.

Common traps

• Choosing KO because its gap of 4 looks right, without checking it is the very next term

No directly related past PYQ was found.