What should come in the place of (?) in the given series?
ACE, FGH, ?, PON

Let's analyze the position of each letter in the alphabetical series ($A=1,B=2,C=3,\dots$):
* First letters of each term:

$$A(1)\stackrel{+5}{\to }F(6)\stackrel{+5}{\to }K(11)\stackrel{+5}{\to }P(16)$$

* Second letters of each term:

$$C(3)\stackrel{+4}{\to }G(7)\stackrel{+4}{\to }K(11)\stackrel{+4}{\to }O(15)$$

* Third letters of each term:

$$E(5)\stackrel{+3}{\to }H(8)\stackrel{+3}{\to }K(11)\stackrel{+3}{\to }N(14)$$

Combining these values, the missing term is KKK.