Final answer:
Biotite is the best rock to represent a pelitic protolith due to its common occurrence in metamorphosed shale, indicating moderate to high-grade metamorphism typical of pelites.
Step-by-step explanation:
The best rock to represent a pelitic protolith would be biotite. This is because during the metamorphism of a shale protolith, which is rich in clay minerals and typically forms in a sedimentary environment, new minerals that indicate the metamorphic grade can form.
Biotite, a mica mineral, is a common product of metamorphism in pelitic rocks along with other minerals such as chlorite, garnet, and several types of alumino-silicate minerals like kyanite, sillimanite, and andalusite. The presence of biotite suggests that the rock has undergone moderate to high-grade metamorphism typical of pelites. This answer excludes options like quartz arenite, which is a type of sandstone, limestone, which is a carbonate rock, and tonalite, an intrusive igneous rock. The option 'NOA' (Nephrite, Omphacite, and Albite) refers to a different category of metamorphic rock.