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

Let's track the shifting pattern for each letter position in the sequence:
1. First Letter:
$\text{F (6)}\stackrel{+1}{\to }\text{G (7)}\stackrel{+1}{\to }\text{H (8)}\stackrel{+1}{\to }\text{I (9)}$
2. Second Letter:
$\text{D (4)}\stackrel{-1}{\to }\text{C (3)}\stackrel{-1}{\to }\text{B (2)}\stackrel{-1}{\to }\text{A (1)}$
3. Third Letter:
$\text{K (11)}\stackrel{+2}{\to }\text{M (13)}\stackrel{+2}{\to }\text{O (15)}\stackrel{+2}{\to }\text{Q (17)}$
4. Fourth Letter:
$\text{I (9)}\stackrel{-2}{\to }\text{G (7)}\stackrel{-2}{\to }\text{E (5)}\stackrel{-2}{\to }\text{C (3)}$

Combining all the positions, the missing cluster is HBOE.