Final answer:
A coarse-grained rock predominantly composed of plagioclase, pyroxene, and olivine, which cools slowly beneath the Earth's surface, is called gabbro.
Step-by-step explanation:
A coarse-grained rock composed predominantly of the minerals plagioclase, pyroxene, and olivine is called gabbro. These mineral contents are indicative of a mafic rock, which is generally dark in color and composed of heavier silicate minerals and rich in magnesium and iron. Gabbro is an intrusive or plutonic rock, meaning it formed from magma that cooled slowly beneath the Earth's surface, allowing the crystals to grow large and interlock.