Work through how coal actually forms and compare it with each rock category.
Metamorphic rocks form when existing rocks are altered by heat and pressure, igneous rocks form when molten magma or lava cools and solidifies, and glacial origin rocks form through the grinding and depositing action of glaciers. Coal does not form by any of these routes.
Coal begins as plant debris in swampy, low oxygen conditions, which gets buried under layers of sediment, compacted, and slowly converted into peat, then lignite, then coal, through a process identical in nature to sedimentary rock formation.
\[\boxed{\text{Sedimentary type}}\]