Which group should replace the question mark in the series A3D, C8H, E15L, G24P, ?

The leading letters run A, C, E, G — two apart — so the next is I (9th). The trailing letters run D, H, L, P — four apart — so the next is T (20th). The numbers are 3, 8, 15 and 24, which are $2^2-1,3^2-1,4^2-1$ and $5^2-1$, so the next is $6^2-1=35$. The group is I35T. I35S takes the trailing letter only three ahead of P; I34T reads the numbers as growing by ten rather than by the squares; K35T moves the leading letter four ahead of G.