The correct answer is D. The electric current. An electromagnet is a type of magnet that is created by running an electric current through a coil of wire. The strength of the magnetism produced by the electromagnet can be increased by increasing the current running through the wire. To make the electromagnet work or stop, you can turn the current on or off using a switch or other device that controls the flow of electricity in the circuit. The other parts of a circuit, such as the ammeter (A) and the connecting wires (B), do not have any direct effect on the electromagnet itself. The magnetism (C) is produced by the movement of electrons in the wire, which is controlled by the electric current.