The correct answer is option 3: diamond is a non-conductor and hard.

Diamond is a form of carbon and is known for its extreme hardness. It is one of the hardest substances on Earth, making it highly resistant to scratching or damage. This hardness is due to the strong covalent bonds between carbon atoms in the crystal lattice structure of a diamond.

When it comes to conductivity, diamond is a non-conductor or insulator. Unlike metals, which have free electrons that can carry electric charge and allow for the flow of electricity, diamond does not have free electrons. It lacks the ability to conduct electricity because the carbon atoms in the diamond`s structure are tightly bonded together, leaving no free or mobile electrons to carry an electric current.

Therefore, diamond is not only extremely hard but also a poor conductor of electricity, making option 3 the correct answer.