Which of the following is NOT an example of metamorphic rock?

Siltstone is the correct answer because it is classified as a sedimentary rock, not a metamorphic rock. Sedimentary rocks are formed from the accumulation of sediment, which can include minerals, organic matter, and other particles deposited over time. In contrast, metamorphic rocks, such as marble, schist, and gneiss, originate from the transformation of existing rock types through intense heat, pressure, and chemical processes beneath the Earth's surface.

Marble forms from the metamorphism of limestone, schist is a foliated metamorphic rock that can originate from shale, and gneiss is created from the metamorphism of granite or other rocks. Each of these examples demonstrates the characteristics and processes specific to metamorphic rocks, which differ fundamentally from the formation of siltstone.