The correct option is A Electromagnet
The doorbell is a useful application of electromagnets.
When a person pushes the button, the electric circuit is closed and current flows. The coil inside the electromagnet is activated and it attracts a magnetic conductor in the bell causing the hammer in the bell to hit the gong and produce a sound.
The bell is designed in such a way that this breaks the circuit and a spring brings it back to the initial position. Again, the circuit is closed. This repeats continuously and the hammer hits the gong multiple times as long as the switch remains closed.