__________________ is a good conductor of electricity. 

Good electrical conductors include graphite, copper, aluminium, silver, and gold. Plastic, rubber, glass, air, wood, charcoal, and diamond are all poor carriers of electricity.
When voltage is applied, a material is said to be an electrical conductor if electrical charge carriers—typically electrons—move smoothly from atom to atom. An illustration of an excellent conductor is: The best electrical conductivity that is commonly used is pure elemental silver. Brass, aluminium, steel, gold, copper, and steel are all effective conductors.
Any material must permit free electron movement in order to be a good conductor of electricity.

Simple movement of free electrons within a matter is what constitutes electric current. A good electrical conductor is one in which the negatively charged electrons that carry the current in it are present.

Hence, the correct answer is option B.