The letters A through D represent different types of volcanic activity - mid-ocean ridges, subduction zones, collision zones between continental plates, and hotspots within a plate.
The letters A through D in the image represent different types of volcanic activity. These types include:
A: Volcanic activity at mid-ocean ridges.
B: Volcanic activity at subduction zones.
C: Volcanic activity at collision zones between continental plates.
D: Volcanic activity caused by hotspot activity within a plate.