Final answer:
The process by which metamorphic rock becomes igneous rock is melting completely, then cooling.
Step-by-step explanation:
The process by which metamorphic rock becomes igneous rock is D. melting completely, then cooling.
Metamorphic rocks form when pre-existing rocks are altered by heat and pressure. However, in order for metamorphic rock to become igneous rock, it must undergo complete melting to form magma or lava. This molten material then cools and solidifies, forming igneous rock.