A particular solid is very hard and has a high melting point. In solid state, it is a non-conductor and its melt is a conductor of electricity. Classify the solid.

The properties given in the question are- 1. Solid is very hard and has high melting point. 2. It is a non-conductor in solid state and conductor in liquid state. These properties tell that it has strong intermolecular forces. We can see that example like NaCl follows all these properties and is an ionic solid. Hence, it is an ionic solid. Thus, the correct option is (d) Unit Cell And Lattices