The correct answer is: A. Mild steel
Mild steel is a type of steel that is low in carbon content. It is a strong and ductile material that is easy to work with. It is also relatively inexpensive, making it a popular choice for brush rockers.
Copper is a good conductor of electricity and heat, but it is not as strong as mild steel. It is also more expensive than mild steel.
Aluminium is a lightweight metal that is strong and corrosion-resistant. However, it is not as strong as mild steel and it is more expensive.
Cast-iron is a strong and durable material, but it is also heavy and brittle. It is not a good choice for brush rockers because it is more likely to break than mild steel.
In conclusion, mild steel is the best material for brush rockers because it is strong, ductile, easy to work with, and relatively inexpensive.