The correct answer is: Diamond.
In diamond, each carbon atom is bound to four other carbon atoms that form a strongly three-dimensional structure. This structure is very strong and makes diamond one of the hardest materials known. Diamond is also a good conductor of heat and electricity.
Ethane is a hydrocarbon with the chemical formula $C_2H_6$. It is a colorless, odorless gas that is found in natural gas and petroleum. Ethane is used as a fuel and as a starting material for the production of other chemicals.
Methane is a hydrocarbon with the chemical formula $CH_4$. It is the simplest alkane and the main component of natural gas. Methane is a colorless, odorless gas that is lighter than air. It is used as a fuel and as a starting material for the production of other chemicals.
Graphite is a form of carbon that is made up of layers of carbon atoms that are arranged in a hexagonal lattice. Graphite is a soft, black material that is used as a lubricant, a conductor of electricity, and a component of pencils.