Which of the following materials is considered a good insulator of electricity?

Plastic is considered a good insulator of electricity because it has a high resistivity, meaning it does not allow electric current to flow easily through it. This property makes plastic an ideal material for insulating wires and electrical components, preventing unintended conduction that could lead to short circuits, electrical shocks, or other hazards.

Unlike metals, which are excellent conductors of electricity due to the presence of free electrons that can move easily, plastic acts as a barrier to electrical flow. Water, on the other hand, is a conductor, especially if it contains impurities, and concrete can conduct electricity when wet or in certain conditions, making both unsuitable as insulators. Therefore, plastic is the clear choice for effective electrical insulation.