Which of the following substances is classified as an insulator?

Porcelain is classified as an insulator because it has high electrical resistivity, which prevents the flow of electric current. Insulators are materials that do not conduct electricity well, and porcelain is commonly used in electrical applications such as insulators for power lines and other electrical components. Its ability to withstand high voltage without conducting electricity makes it an essential material in electrical and electronic equipment.

In contrast, aluminum, copper, and iron are all conductors of electricity. These metals have free electrons that allow them to easily carry an electrical current, which is essential for their use in wiring and other electrical applications. Their conductive properties are why they are typically employed in electrical installations, rather than insulatory functions.