Correct option is A
Soft iron is the best choice for the core of an electromagnet:
Magnetic properties
When an electric current passes through the coil of an electromagnet, the soft iron core becomes magnetic and strengthens the magnetic field. When the current stops, the soft iron loses its magnetism.
Magnetizing and demagnetizing properties
Soft iron has the best magnetizing and demagnetizing properties of the materials commonly used to make electromagnets, including stainless steel, silver, and nickel.
