Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.
ACE, DFH, GIK, JLN, ?

The correct option is MOQ.

To find the missing letter-cluster that replaces the question mark (?), let us analyze the pattern followed by the letters in each position across the given series:

Series: ACE, DFH, GIK, JLN, ?

1. First letter of each cluster:
A → D → G → J → ?
- A + 3 = D
- D + 3 = G
- G + 3 = J
- J + 3 = M

2. Second letter of each cluster:
C → F → I → L → ?
- C + 3 = F
- F + 3 = I
- I + 3 = L
- L + 3 = O

3. Third letter of each cluster:
E → H → K → N → ?
- E + 3 = H
- H + 3 = K
- K + 3 = N
- N + 3 = Q

Combining the resulting letters from each step gives MOQ.