Which letter cluster will replace the question mark (?) to complete the given series?
ABCD, EEEE, ?, MKIG, QNKH

Let's examine the progression of each letter position across the clusters:
1st Letter: A (+4) → E (+4) → I (+4) → M (+4) → Q

2nd Letter: B (+3) → E (+1) → F (+5) → K (+3) → N

Let's look more closely at the second letters: B(2), E(5), ? , K(11), N(14).
Notice the gap between E(5) and K(11) is 6. If we split it evenly with +3, the middle letter position becomes 5+3=8, which is H. Let's verify: B(2) +3 → E(5) +3 → H(8) +3 → K(11) +3 → N(14). This works perfectly! So the 2nd letter is H.

3rd Letter: C (+2) → E (0) → G (-2) → I (0) → K? No, let's re-verify the values: C(3), E(5), ?, I(9), K(11). This is a simple progression of odd numbers/adding 2: 3→5→ 7 (G) →9→11. So the 3rd letter is G.

4th Letter: D(4), E(5), ?, G(7), H(8). This is consecutive sequential addition (+1): 4→5→ 6 (F) →7→8. So the 4th letter is F.

Combining these positions gives IHGF.