Answer:
D. Hot mantle plumes rise toward the surface within a plate
Step-by-step explanation:
Intraplate volcanism happens inside a plate, not at a plate boundary. 
Thus, Options B and C are wrong. 
In intraplate volcanism, a hot mantle plume slowly rises by convection from the lower mantle and creates a hot spot at the base of a plate, penetrating the crust and causing localized volcanic activity. 
A. is wrong. Descending magma would not cause volcanic activity .