The right answer is protolith.
The protolith is an original rock of a metamorphic rock.
Metamorphism is the set of processes that induce mineralogical and textural modifications of a rock, in the solid state and under the effect of temperature, pressure and fluids.
When there is no longer any indication of the nature of the original rock, it is sometimes possible to identify it from the overall chemical composition of the rock (provided that the chemical changes are not too large).
