Final answer:
Gabbro is the correct answer because it is a low silica rock that cooled slowly deep beneath the Earth's surface, forming a coarse-grained intrusive igneous rock.
Step-by-step explanation:
The low silica rock that cooled slowly is b. Gabbro. Gabbro is an intrusive igneous rock, which means it formed from magma that cooled slowly beneath the Earth's surface. It is coarse-grained, as individual crystals had time and space to grow large, a characteristic of slow cooling. Rocks that cool slowly are generally rich in magnesium and iron, making them darker in color and lower in silica compared to their extrusive counterparts like rhyolite and basalt.
By contrast, rhyolite, although similar in chemical composition to granite, is an extrusive igneous rock and therefore cools quickly on the Earth's surface, forming small crystals. Granite also has a slow cooling process but has higher silica content than gabbro. Basalt, similar to but darker than gabbro, is an extrusive rock that forms from lava that cools quickly on the Earth's surface.