Which rock is predominantly formed from feldspar?

Anorthosite is a type of intrusive igneous rock that is predominantly composed of plagioclase feldspar, specifically the mineral anorthite. This characteristic sets it apart from other rock types, as its composition can be over 90% feldspar, particularly in its purest forms. The abundance of feldspar in anorthosite gives it a light color, often appearing white or light gray.

In contrast, basalt is primarily composed of pyroxene and plagioclase feldspar, but it has a significantly lower feldspar content compared to anorthosite. KREEP, which stands for potassium, rare earth elements, and phosphorus, refers to a specific lunar basaltic rock that contains a variety of minerals but is not predominantly feldspar. Lastly, a comet is not a rock; rather, it is an icy body in space, often composed of dust, rock, and frozen gases, making it entirely unrelated to the composition of terrestrial rocks.

Thus, the defining characteristic of anorthosite being predominantly formed from feldspar makes it the correct answer.