The correct answer is A. mild steel.
Mild steel is a type of steel that is low in carbon content. It is strong and ductile, making it ideal for use in a variety of applications, including the manufacture of ordinary rails.
Cast iron is a type of iron that is made by melting iron ore and then pouring it into a mold. It is strong and brittle, making it less ideal for use in the manufacture of ordinary rails.
Wrought iron is a type of iron that is made by heating iron ore and then hammering it into shape. It is strong and ductile, but it is not as strong as mild steel.
High carbon steel is a type of steel that is high in carbon content. It is strong and hard, but it is also brittle. This makes it less ideal for use in the manufacture of ordinary rails, which need to be able to withstand a lot of wear and tear.
In conclusion, the correct answer is A. mild steel.