Which of the following is NOT a type of rock?

The term "organic" is not recognized as a distinct category of rock in geology. Instead, rocks are classified into three primary types: igneous, sedimentary, and metamorphic.

Igneous rocks form from the cooling and solidification of molten material (magma or lava), and they can be either intrusive (formed beneath the earth’s surface) or extrusive (formed on the surface). Sedimentary rocks are created from the accumulation and compression of mineral and organic particles, often forming in layers over time. Metamorphic rocks arise from the alteration of existing rocks due to heat, pressure, and chemically active fluids, which change their mineral composition and structure without melting them.

While organic materials do play a role in the formation of some sedimentary rocks, such as limestone (which can form from the accumulation of shell debris), the classification of "organic" as a type of rock does not align with the established geological classification system. Thus, it stands out as the correct answer to the question.