An organic rock is a type of rock that is formed from the remains of once-living organisms. Option 1, marble, is not an organic rock because it is formed from the metamorphism of limestone, which is made up of inorganic minerals. Option 3, granite, is also not an organic rock because it is formed from the cooling and solidification of magma or lava, which are inorganic substances. Option 4, slate, is a metamorphic rock formed from the sedimentary rock shale, which is composed of inorganic minerals.

Option 2, coal, is the correct answer because it is formed from the ancient remains of plants that have gone through a process called carbonization. Over time, the organic material compacts and undergoes chemical changes, resulting in the formation of coal. Therefore, coal is considered as an organic rock.