The most suitable material for making the core of an electromagnet is:

Steel Iron Soft iron Aluminium

Soft iron

The soft iron inside the coil makes the magnetic field stronger because it becomes a magnet itself when current is flowing through the coil. Moreover, it loses its magnetism as soon as the current stops flowing. It forms a temporary magnet. In this way, the magnetism induced through electricity can be controlled by switching the ON and OFF button.