Which would come in place of '?' in the given series?
ACE, FGH, ?, PON

Let us analyze the position and structural pattern of each letter group in the alphabetical series:
1. Look at the first letters of each group:

* $\text{A}\stackrel{+5}{\to }\text{F}$
* Following the same rule: $\text{F}\stackrel{+5}{\to }\text{K}$
* Let us check the next term: $\text{K}\stackrel{+5}{\to }\text{P}$
This confirms that the first letter of the missing group is $\text{K}$.

2. Now examine the differences between subsequent letters within each specific group:

* ACE: $\text{A}(+2)\text{C}(+2)\text{E}$ (Inner difference is $+2$)
* FGH: $\text{F}(+1)\text{G}(+1)\text{H}$ (Inner difference is $+1$)
* ?: The inner difference should logically decrease to $0$. Thus, $\text{K}(+0)\text{K}(+0)\text{K}⟹\text{KKK}$.
* PON: $\text{P}(-1)\text{O}(-1)\text{N}$ (Inner difference is $-1$)

This establishes a consistent decreasing pattern of internal letter spacing: $+2\to +1\to 0\to -1$. Therefore, the missing group is $\text{KKK}$.