Which of the following is a good electrical insulator?

An electrical insulator is a material whose internal electric charges do not flow freely, making it very hard to conduct an electric current. Glass is a classic example of an excellent insulator and is widely used in electrical applications, such as power line standoffs, to prevent current from flowing where it shouldn't. In contrast, copper, aluminum, and mercury are all metals. Metals have free electrons that make them excellent conductors of electricity, which is the exact opposite of an insulator.