Which of the following materials is primarily found on black lunar soil?

Basalt is the correct answer because it is a volcanic rock that forms as a result of lava cooling and solidifying. The Apollo missions returned samples of lunar soil, also known as regolith, which displayed a high concentration of basalt, particularly in the darker areas of the Moon known as the maria. These basaltic areas were formed by ancient volcanic activity and have a smoother texture compared to the lighter regions of the Moon, which are predominantly composed of anorthosite, another type of rock.

Basalt’s primary mineral composition typically includes plagioclase, pyroxene, and olivine, contributing to its dark coloration. This is why the black lunar soil is characterized mainly by basalt. Other materials like KREEP, or potassium, rare earth element, and phosphorus-rich materials, are also present but not as dominant in the lunar surface's black soil. Anorthosite is predominantly found in the highlands of the Moon and consists mostly of plagioclase feldspar, while pyroxferroite, a mineral containing iron, is not a major component of lunar soil.