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

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

Combining these letters yields the missing cluster: HBOE.