Answer
b. metal
Explanation
Metals are the best conductors of electricity. Metals have free electrons that are responsible for carrying current. The conductivity in metals also differ and we cannot be absolutely correct to say that all metals are good conductors. The best metals used for conductivity of current are, copper, silver and gold.
Glass, plastic, rubber and wood are poor conducers of both current and heat since they do not have free electrons to conduct the current or even heat.
