Concept:Insulators have very few free charge carriers, so their electrical conductivity is very low but not exactly zero.Explanation:Insulators do not allow electric current to flow easily because free electrons are almost absent in them.However, the conductivity is not exactly zero in practical conditions.A very tiny current can pass through an insulator under high voltage or high temperature.Therefore, the electrical conductivity of insulators is extremely small, but not zero.Answer:Option C: extremely small