Which processes form igneous rocks? A. heat and pressure B. pressure and cooling C. melting and cooling D. melting and pressure

The answer is "C", and then you have two different types of igneous rocks, one being extrusive (magma that cools faster, and cools on/near the surface, results in obsidian, basic rock, etc), and intrusive (magma that cools slowly and is inside the earth, forms diamonds and other gems of high value and appearance).