We are asked to identify what type of rock would be formed in a plant-rich swamp. We all know that plant dies and the remain leaves in the soil called peat. Temperature, pressure and heat causes the remains to submerged under the soil bed layer and converts it into a waxy substance called kerogen. Kerogen then mixes with the rock under the soil bed layer to create what is called an oil shale. Oil shale is somehow similar to a coal. Both creates an energy when ignited. Therefore the correct answer to the question being asked is COAL or Oil Shale.