Arock that is intermediate between slate and schist is
a. skarn
b. gneiss
c. phyllite
d. quartzite

Option (C)

Explanation:

Slate is a metamorphic rock which is characterized by the presence of fine grains that forms foliated structure.

Schist is a high-grade metamorphic rock which commonly characterized by the presence of foliated minerals.

The above-given condition describes the grade of metamorphism, where a low-grade metamorphic rock changes into a high-grade metamorphic rock due to the increase in the temperature and the pressure condition.

Slate is a low-grade metamorphic rock that changes into another metamorphic rock named Phyllite and because of the further increase in the temperature-pressure condition, it changes into a higher grade metamorphic rock named schist.

Phyllite is a metamorphic rock that is mainly comprised of minerals such as quartz, mica, and some chlorite.

Thus, the correct answer is option (C).