Correct Answer: Coal

Solution : The correct answer is Coal.

Coal is formed by the process of carbonation in which organic material i.e. dead plants and trees are burnt deep inside the earth under high pressure and temperature for million of years. It can be considered as an organic rock. Because dead plants and trees are layered by sediments over a long period of time i.e. why coal deposits are usually found in sedimentary rocks. Slate, marble and granite are metamorphic rocks which are formed by the rearrangement of molecules and they are inorganic rocks.