Final answer:
The protolith subjected to metamorphism may be any one of the three primary rock types, which includes igneous, sedimentary, or metamorphic rocks.
Step-by-step explanation:
The protolith subjected to metamorphism can be any type of the three primary rocks, which are igneous, sedimentary, or metamorphic. The protolith is known as the parent rock from which a metamorphic rock forms. Metamorphism is the process by which a protolith is altered by a change in temperature, pressure, or contact with chemically reactive fluids.
All rocks are part of what's known as the rock cycle, a concept in geology that describes the transitions among the three main rock types. Therefore, a protolith that undergoes metamorphism can initially be an igneous rock that cools from magma or lava, a sedimentary rock formed from weathered remnants of older rocks and biological material, or even a metamorphic rock that experiences further metamorphic processes.