Which metamorphic rocks are most likely to have formed at the highest temperatures and pressures?

Answer: Schist and gneiss

Explanation: The rocks that forms at the highest pressure and temperature condition are Schist and Gneiss, where Gneiss being the highest grade metamorphic rock.

Schist is mainly comprised of quartz, some feldspars and platy minerals such as biotite and muscovite and are foliated rocks with a distinct cleavage commonly known as the schistocity.

Gneiss is mostly composed of quartz, hornblende, feldspar and mica. It has a distinct alternating bands of light and dark colored minerals. The minerals in the darker band contains hornblende, mica and garnet. The lighter band contain minerals such as feldpar and quartz. This banding has a significant characteristic for defining a gneissic rock.

gneiss is formed at the highest temperatures and pressures