Volcanoes are formed question 3 options:
a. when the earth's core erupts onto the surface
b. where the earth's crust is especially thick
c. in subduction zones or in rift valleys
d. after earthquakes have damaged the earth's crust
e. as a result of water seeping into the core from the surface

I think the correct answer is C. Volcanoes are formed in subduction zones or in rift valleys. They are formed when hot magma within the upper mantle of the Earth would work way up the surface. As it reaches the surface, an eruption would happen forming lava flows and deposits of ash. As it continues to erupt, it would grow bigger and bigger. These would form commonly in divergence or convergence tectonic plates. On Earth, there are about 1500 volcanoes that are active.  About 80 of these can be found under the oceans. The active ones are mostly found in washington, oregon, alaska, california and hawaii.