The correct answer is A. Diamond.
Diamond is the hardest natural substance on Earth. It is an allotrope of carbon, which means that it is a form of carbon that has a different atomic structure than other forms of carbon, such as graphite and coal. Diamond is made up of carbon atoms that are arranged in a tetrahedral structure. This structure gives diamond its strength and hardness.
Graphite is another allotrope of carbon. It is made up of carbon atoms that are arranged in a hexagonal structure. Graphite is a soft material that is used in pencils and lubricants.
Coke is a solid fuel that is made from coal. It is made by heating coal in the absence of air. Coke is a hard material that is used in blast furnaces to make iron and steel.
Lamp black is a fine black powder that is made from soot. It is used in paints, inks, and other products.
Diamond is the hardest allotrope of carbon because of its tetrahedral structure. The tetrahedral structure is very strong and makes it difficult to break the bonds between the carbon atoms. This makes diamond the hardest natural substance on Earth.