Amass of rock formed when a large body of magma cools inside the crust is called a

The correct answer is - batholith.

The batholith represents a very large mass of intrusive igneous rocks that occupy an area of more than 100 square kilometers. It forms deep inside the Earth's crust from cooled magma, and they are structures with very complex compositions and history behind them. Predominantly, batholiths are made out of intermediate or/and felsic rock types, like the likes of diorite, granite, and quartz monzonite.