The correct answer is D. Peat.
Peat is the least mature type of coal and is formed when plant material is buried and subjected to heat and pressure over time. It is a soft, brown material that contains recognizable traces of the original plant material.
Bitumen is a type of coal that is intermediate in maturity between peat and anthracite. It is a black, shiny material that is not as hard as anthracite.
Anthracite is the most mature type of coal and is a hard, black material that does not contain any recognizable traces of the original plant material.
Lignite is a type of coal that is intermediate in maturity between peat and bitumen. It is a brown, soft material that is not as hard as bitumen.
Peat is the only type of coal that contains recognizable traces of the original plant material. This is because it is the least mature type of coal and has not been subjected to as much heat and pressure as the other types of coal.