Select the letter-cluster from among the given options that can replace the question mark (?) in the following series. DAG, FCI, ? , JGM, LIO

Correct — D.
Rule: +2 at every position, term to term.

First letters: D, F, ?, J, L → H(8).
Second letters: A, C, ?, G, I → E(5).
Third letters: G, I, ?, M, O → K(11).

That gives HEK, and HEK + 2 = JGM checks it forward → option (d).

Why the others are wrong

• (a)
  GDJ — GDJ adds 1 to each letter of FCI instead of 2. It would then sit 3 short of JGM, so the step would not stay constant.
• (b)
  IFL — IFL adds 3 to each letter of FCI and sits only one step short of JGM. The series would jump +3 then +1 instead of +2 throughout.
• (c)
  HKE — HKE holds the right letters in the wrong order. The middle position runs A, C, ?, G, so it needs E, and the last runs G, I, ?, M, so it needs K.

A letter-cluster series with a gap in the middle is solved the same way as one with a gap at the end. Convert to positions and read each position down the terms.

Here all three positions step +2: D F H J L, A C E G I, G I K M O. Because the missing term has a known neighbour on both sides, you can check it both ways: FCI + 2 gives HEK, and HEK + 2 gives JGM.

Key facts

• D = 4, F = 6, H = 8, J = 10, L = 12: the first letters climb by 2.
• A missing middle term must fit both the term before it and the term after it.
• The same letters in a different order make a different cluster.

Common traps

• Choosing HKE because it holds H, E and K in some order
• Checking the missing term against FCI only and never against JGM

No directly related past PYQ was found.