A non-metal which is a good conductor of electricity is
A. Graphite
B. Hydrogen
C. Sodium
D. Mercury

Hint: It is an allotrope of carbon. It is a metalloid, most stable form of carbon under standard conditions (273.15K and 1atm). Other name of this compound is Black lead (this name came from its use in pencils)

Complete step by step answer:
- Graphite is a good conductor of electricity, though it is a non-metal (specifically semi-metal).
- A substance will be a good conductor of electricity if it has free electrons in its valence shell. As we know, carbon is tetravalent (it has a valency 4, being able to form four single covalent bonds).
- In graphite, the carbon atoms are arranged in a manner of several sheets of hexagonal rings that are held together by weak Vander Waals forces. Each carbon atom in the arrangement is bonded with three other carbon atoms, hence leaving one free electron in the valence shell, making it a good conductor.
- Hydrogen is a non-metal, but good conductor of electricity only at very high pressure. Thus, it cannot be a correct answer.
- Sodium is a good conductor of electricity, but is an s-block element and is categorized as alkali metal. So, it is not a non-metal. Thus, it is also not a correct answer.
- Mercury is also a metal. So, it can also not be the true answer.

So, the correct answer is “Option A”.

Note: Remember that metals are not found in liquid state but there is an exception in case of Mercury which is a liquid metal. So, do not consider mercury as a non-metal element.