The correct answer is: A. Natural graphite
Natural graphite is a soft, brittle material that is easily worn away. This makes it unsuitable for use in brushes that come into contact with commutators, as it can cause excessive wear of the commutator.
Hard carbon is a more durable material than natural graphite, but it is still not as wear-resistant as electrographitic or metal graphite brushes. Electrographitic brushes are made from a mixture of graphite and metal powder, which gives them a higher degree of hardness and wear resistance. Metal graphite brushes are made from a mixture of graphite and metal, which gives them the highest degree of hardness and wear resistance of all the options.
In summary, natural graphite is the least wear-resistant material, followed by hard carbon, electrographitic brushes, and metal graphite brushes.