A hard substance melts at high temperature and is an insulator in both solid and in molten state. This solid is most likely to be a / an :

A hard substance that melts at a high temperature and is an insulator in both the solid and molten states must be a covalent (network) solid. Ionic solids are hard with high melting points but conduct electricity in the molten state due to free ions. Molecular solids are soft with low melting points. Metallic solids conduct electricity in both states.

Covalent solids such as diamond and silicon carbide are extremely hard, have very high melting points, and do not conduct electricity in either state because all electrons are locked in strong covalent bonds with no free charge carriers.

The correct answer is option (4): Covalent solid.