The correct answer is: C. Diamond is the hardest, graphite is the softest.
The other options are incorrect because:
- Coke is not an allotrope of carbon. It is a solid form of carbon that is produced by heating coal in the absence of air.
- Lamp black is a fine black powder that is produced by incomplete combustion of carbonaceous materials. It is not an allotrope of carbon.
Graphite is a soft, black, lustrous form of carbon that is used in pencils and lubricants. It is made up of layers of carbon atoms that are arranged in a hexagonal lattice. The layers are held together by weak van der Waals forces, which allows them to slide past each other. This makes graphite a good lubricant.
Diamond is a hard, colorless, transparent form of carbon that is the hardest natural substance on Earth. It is made up of carbon atoms that are arranged in a tetrahedral lattice. The strong covalent bonds between the carbon atoms make diamond very hard.
The hardness of a material is its resistance to deformation, such as scratching or indentation. The Mohs scale of mineral hardness is a qualitative scale that ranks the scratch resistance of various minerals. Diamond is the hardest mineral on the Mohs scale, with a hardness of 10. Graphite is the softest mineral on the Mohs scale, with a hardness of 1.