The correct term to finish the statement is "protolith."
Here's an explanation:
1. Limestone is a sedimentary rock that can undergo metamorphism to become marble.
2. In the context of metamorphic rocks like marble, the term "protolith" refers to the original rock from which it was formed.
3. Therefore, in the given statement, the term "protolith" appropriately describes the relationship between limestone and marble in the rock cycle.
