Final answer:
Volcanoes are formed at divergent and convergent plate boundaries due to processes like the rising of magma at mid-ocean ridges and the subduction of oceanic crust beneath continental crust.
Step-by-step explanation:
Volcanoes are primarily formed by two types of plate boundaries: divergent and convergent. At divergent boundaries, tectonic plates move apart from each other, leading to the formation of new crust as magma rises and cools at the surface, creating volcanic activity. This is evident at mid-ocean ridges, where undersea volcanic mountains are formed. Convergent boundaries, on the other hand, occur when plates move towards each other. In cases where an oceanic plate and a continental plate collide, the denser oceanic plate subducts beneath the continental one. This subduction process can lead to the melting of the subducted plate and the subsequent rise of magma to form volcanoes.
Transform boundaries usually do not result in volcanism because they involve two tectonic plates sliding past each other without the creation or subduction of crust. While earthquakes are common along transform faults, such as the San Andreas Fault, they do not typically lead to the formation of volcanoes. Therefore, the final answer to which two plate boundaries form volcanoes is A) Divergent, Convergent.