What is defined as a substance that does not easily transmit electricity?

The correct answer is based on the definition of materials in terms of electrical conductivity. An insulator is a substance that resists the flow of electric current, meaning it does not easily transmit electricity. This property is due to the structure of the material, which has tightly bound electrons and a lack of free-moving charges that can carry an electrical charge.

In contrast, a conductor allows electricity to flow freely due to the presence of free electrons. Conductors are essential in electrical systems, as they facilitate the transfer of electrical energy.

A resistor is a component that limits the flow of electric current, but it does so within a circuit and is not classified strictly as a substance that does not transmit electricity as an insulator does.

A capacitor is a device used to store electrical energy and does not fit the definition provided in the question regarding the transmission of electricity.

Thus, identifying the properties of insulators clarifies their role in preventing electrical transmission, making them the correct answer to the question.