Insulator:

An insulator is a material that does not allow the flow of electric current. It has high electrical resistance and low conductivity. In the given options, the correct answer is option 'C', which is diamond.

Diamond as an insulator:

Diamond is a form of carbon in which the carbon atoms are arranged in a crystal lattice structure. It is a well-known fact that diamond is an excellent conductor of heat due to its high thermal conductivity. However, when it comes to electricity, diamond behaves as an insulator.

Explanation:

1. Diamond Structure:
Diamond has a unique crystal lattice structure in which each carbon atom is covalently bonded to four neighboring carbon atoms. This results in a three-dimensional network of carbon atoms. The strong covalent bonds between the carbon atoms make diamond a very hard material.

2. Band Gap:
The electrical conductivity of a material depends on its band structure. In the case of diamond, the valence band is fully occupied with electrons, and there is a large energy gap between the valence band and the conduction band. This energy gap is known as the band gap.

3. Band Gap of Diamond:
The band gap of diamond is approximately 5.5 eV (electron volts). This means that a significant amount of energy is required to promote an electron from the valence band to the conduction band. As a result, diamond does not conduct electricity easily.

4. Insulating Properties:
Due to the large band gap, diamond does not have enough free electrons or electron holes to carry an electric current. When an electric field is applied, the electrons in the valence band do not have enough energy to jump to the conduction band, and thus, the flow of electric current is restricted.

Conclusion:

Diamond's unique crystal structure and large band gap make it an insulator with respect to the flow of electric current. Despite its excellent thermal conductivity, diamond cannot conduct electricity easily due to its electronic properties.