The correct answer is: C. Electro-graphitic brushes.
Electro-graphitic brushes are made of a mixture of graphite and metal powder. They are very hard and have a high electrical conductivity, which makes them ideal for use in applications with high values of current density.
Metal graphite brushes are made of a mixture of graphite and metal powder. They are less hard than electro-graphitic brushes, but they have a higher thermal conductivity. This makes them ideal for use in applications where the brushes are exposed to high temperatures.
Hard carbon brushes are made of pure carbon. They are very hard and have a high electrical conductivity, but they have a low thermal conductivity. This makes them ideal for use in applications where the brushes are not exposed to high temperatures.
Natural graphite brushes are made of pure graphite. They are soft and have a low electrical conductivity. This makes them ideal for use in applications where the brushes are not exposed to high values of current density.
In conclusion, electro-graphitic brushes are the best choice for applications with high values of current density. They are hard, have a high electrical conductivity, and are resistant to wear.