Which of the following is a primary rock-forming mineral?

A mineral is a naturally occurring, inorganic solid with a definite chemical composition and a crystalline structure. A rock, by contrast, is an aggregate of one or more minerals. The question asks for a mineral, so any option that names a rock rather than a mineral must be eliminated.

Basalt is an igneous rock, not a mineral. It is composed of several minerals (primarily plagioclase feldspar, pyroxene, and sometimes olivine), so it does not qualify as a rock-forming mineral.

Quartz, feldspar, and mica are all genuine minerals and all qualify as primary rock-forming minerals. Among them, feldspar is the most abundant mineral group in the Earth's crust, making up roughly 60% of it. Because of this dominance, feldspar is the classic example cited when a single primary rock-forming mineral is requested. It occurs in igneous, sedimentary, and metamorphic rocks alike, and its chemical weathering is a major source of clay minerals in soil.

While quartz and mica are also rock-forming minerals, feldspar's overwhelming abundance and its role in soil formation make it the strongest answer in an agricultural-science context.