Which of the following is a good electrical insulator?

Glass is considered a good electrical insulator because it has a very high resistivity, meaning it does not allow electrical current to flow easily. This characteristic is due to the lack of free electrons in the glass structure; it consists mainly of tightly bonded atoms, which do not permit the movement of electrical charge. Insulators like glass are essential in various electrical applications, as they help prevent unintended flow of current and protect against electrical shocks.

In contrast, materials such as copper and aluminum are conductors, meaning they allow electricity to flow freely due to the availability of free electrons. Mercury, being a metal as well, also conducts electricity. Therefore, glass stands out in this context because of its properties that inhibit electrical conductivity, making it suitable for use in applications requiring electrical insulation.