Which is a coarse-grained, igneous rock composed mainly of pyroxene, plagioclase feldspar, and olivine?

The answer is gabbro.

Gabbro is a group of rocks that are igneous and phaneritic (coarse-grained). It is formed when magma trapped beneath the surface begins to cool slowly. It consists of pyroxene, feldspar, plagioclase, and, in smaller ratio, olivine. Gabbro's common name is 'black granite'.