Which out of the following letters should replace the question mark in the series?
A, D, G, J, M, ?

Let's track the numerical alphabetical positions of the letters given in the series:
* $\text{A}=1$
* $\text{D}=4$
* $\text{G}=7$
* $\text{J}=10$
* $\text{M}=13$

The difference between any two consecutive letters is $+3$.
Following this arithmetic sequence, the next position value is:

$$13+3=16$$

The 16th letter of the English alphabet is P.