'Coal' is an example of

Rocks are classified into three groups based on how they form: igneous rock forms from the cooling and solidification of molten magma or lava; metamorphic rock forms when an already-existing rock is altered by heat and pressure without melting; and sedimentary rock forms through the compaction, cementation, and lithification of deposited material -- including organic remains -- over long geological time.

Coal originates from dead plant matter that accumulates in swampy environments. Over millions of years this organic matter is buried under successive layers of sediment; the combined effect of burial pressure and heat (a process geologists call coalification) compacts and lithifies it, converting peat into lignite, then bituminous coal, and eventually anthracite. Because coal forms by the burial and lithification of deposited organic material -- the defining process of sedimentary rock formation -- coal is classified as an organic (biogenic) sedimentary rock.

• Metamorphic rock forms when an already-existing rock is altered by heat and pressure without melting (for example, limestone into marble). Coal as a whole is not formed this way -- it originates from the burial and lithification of deposited organic sediment, which is a sedimentary process; only the highest-rank coal, anthracite, undergoes a further heat-and-pressure overprint on already-lithified coal that some sources describe as metamorphic, but the standard classification of coal itself remains sedimentary.

• Igneous rock requires cooling and solidification of molten magma or lava (for example, granite or basalt); coal has no molten or volcanic origin, so it does not fit this class either.

• Since coal's formation matches the sedimentary (lithification-of-deposited-material) pathway and fits none of the other two, one of the offered rock classes clearly applies to it.

Coal is therefore a sedimentary rock.