The correct answer is: D. Cast iron
Cast iron is a type of iron that has been melted and then poured into a mold to create a desired shape. It is a relatively weak material, but it is also very strong in compression. This makes it ideal for use in field frames, which need to be able to withstand a lot of weight. Cast iron is also relatively inexpensive, which makes it a good choice when cost is a major factor.
Aluminium is a lightweight metal that is often used in construction and manufacturing. It is strong and durable, but it is also relatively expensive. Soft steel is a type of steel that is not as strong as other types of steel, but it is also more malleable. This makes it ideal for use in applications where it needs to be able to be shaped easily. Cast steel is a type of steel that is made by melting iron and then pouring it into a mold. It is a strong and durable material, but it is also relatively heavy.
In conclusion, cast iron is the best material for field frames because it is strong in compression, relatively inexpensive, and easy to work with.