The correct answer is C. Peat.
Peat is the lowest rank of coal, formed when partially decomposed plant material is buried under layers of sediment and subjected to heat and pressure over millions of years. It is a dark, carbon-rich material that can be used as a fuel or as a soil amendment.
Lignite is a higher rank of coal than peat, formed when peat is buried deeper and subjected to more heat and pressure. It is a brown, soft coal that is used as a fuel in power plants.
Anthracite is the highest rank of coal, formed when lignite is buried even deeper and subjected to even more heat and pressure. It is a hard, black coal that is used as a fuel in power plants and in the production of steel.
Bitumen is a type of asphalt that is found in oil shale and tar sands. It is a thick, black liquid that is used to make asphalt pavement and other products.
Peat is the only variety of coal that contains recognizable traces of the original plant material. This is because it is the lowest rank of coal and has not been subjected to as much heat and pressure as the other varieties of coal.