The correct answer is B. Cast steel.
Cast steel is a type of steel that is cast into a mold. It is used in a variety of applications, including those that require superior mechanical properties. Cast steel is strong and durable, and it can withstand high temperatures and pressures. It is also resistant to corrosion and wear.
Grey cast-iron is a type of cast iron that is composed of carbon, silicon, and iron. It is used in a variety of applications, including those that require good machinability and thermal conductivity. Grey cast-iron is strong and durable, but it is not as strong as cast steel. It is also more brittle and more susceptible to corrosion.
High carbon steel is a type of steel that contains a high percentage of carbon. It is used in a variety of applications, including those that require high strength and hardness. High carbon steel is strong and durable, but it is also more brittle than cast steel. It is also more susceptible to corrosion.
Stainless steel is a type of steel that is resistant to corrosion. It is used in a variety of applications, including those that require good corrosion resistance. Stainless steel is strong and durable, but it is not as strong as cast steel. It is also more expensive than cast steel.
In conclusion, cast steel is the best material for those portions of magnetic circuit which carry steady flux and need superior mechanical properties.