Concept:Coal is a fossil fuel that belongs to the class of rocks formed from living organic matter.
Explanation:Coal originates from the remains of ancient plants that were buried and compressed for millions of years.
The process starts when dead vegetation accumulates in swampy environments.
Over time, heat and pressure convert this plant matter into peat and eventually into coal.
Because the source material is organic, coal is classified as an organically-formed sedimentary rock.
This same rock group includes limestone, which also forms from organic activity.
Mechanically-formed sedimentary rocks are made from broken fragments of older rocks.
Plutonic igneous rocks form from cooling magma deep underground, while hard crystalline rocks are usually igneous or metamorphic.
None of these alternative rock types match the organic origin of coal.
Answer:Option C — Organically-formed sedimentary rocks.