The correct answer is: C. diamond
Diamond is the hardest form of carbon. It is a crystalline allotrope of carbon with its atoms arranged in a cubic lattice. Diamond is one of the most valuable gemstones and is also used in industrial applications such as cutting tools and abrasives.
Coke is a solid fuel made from coal. It is produced by heating coal in the absence of air, which removes the volatile components of the coal. Coke is used as a fuel in blast furnaces to produce iron and steel.
Graphite is a soft, black mineral that is a form of carbon. It is made up of layers of carbon atoms that are arranged in a hexagonal lattice. Graphite is used as a lubricant, a conductor of electricity, and a component of pencils.
Charcoal is a black, porous solid that is produced by heating wood or other organic materials in the absence of air. Charcoal is used as a fuel, a preservative, and a filter.