Which of the following is good insulator?

The correct option is Glass.

An insulator is a material that does not easily allow the flow of electric current or heat through it. This property exists because the electrons in an insulator are tightly bound to their parent atoms and are not free to move around easily. Let us analyze the given options to understand why glass is a good insulator:
1. Mercury is a metal that exists in a liquid state at room temperature. Like other metals, it has free electrons and is a good conductor of electricity.
2. Cadmium is a transition metal. It has free valence electrons, making it a good electrical conductor.
3. Germanium is a metalloid (semiconductor). Its conductivity lies between that of a conductor and an insulator, meaning it can conduct electricity under certain conditions but is not a good insulator.
4. Glass is a non-metallic, amorphous solid. It lacks free electrons to conduct electricity, giving it extremely high electrical resistivity. Therefore, glass is a very good insulator and is widely used for insulation purposes in electrical transmission lines and equipment.