How is casting useful for a sculptor? And
What materials are usually involved in the process?

The method is used mainly for producing solid castings in aluminum that can be welded or riveted together to make the finished sculpture.

The melted down metal or other liquid substance is poured into a mold to harden. This method allows the artist to duplicate an original sculpture done in way, clay, plaster, or some other material.

what are the choices.

option c (nickel) is not found in earth's crust. the earth's crust basically contains eight (8) elements namely: oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium (listed in order of abundance).