Which letter cluster can replace the question mark (?) to complete the given series?
FDKI, GCMG, ?, IAQC

The correct option is HBOE.

To find the missing letter cluster, let us analyze the pattern followed by the letters in each position of the given series: FDKI, GCMG, ?, IAQC.

1st letter of each cluster:
F → (+1) → G
G → (+1) → H
H → (+1) → I
The first letter increases by 1 position (+1) in alphabetical order.

2nd letter of each cluster:
D → (-1) → C
C → (-1) → B
B → (-1) → A
The second letter decreases by 1 position (-1) in alphabetical order.

3rd letter of each cluster:
K → (+2) → M
M → (+2) → O
O → (+2) → Q
The third letter increases by 2 positions (+2) in alphabetical order.

4th letter of each cluster:
I → (-2) → G
G → (-2) → E
E → (-2) → C
The fourth letter decreases by 2 positions (-2) in alphabetical order.

Combining the missing letters obtained from each position (H, B, O, E), we get the letter cluster HBOE.