Question

what caused the volcano (b)? they appear on their own magma melted from continental crust magma from other volcanoes magma from the subduction zone

Explanation:

Volcanoes are caused by magma reaching the Earth's surface. Magma from the subduction - zone is a common source as subduction of oceanic plates can melt the mantle and form magma that rises to create volcanoes. Continental crust melting is less common for typical volcanic activity, and volcanoes don't appear on their own without a source of magma. Magma from other volcanoes is not the fundamental cause.

Answer:

magma from the subduction zone