Electric motor is used to convert:

Electric motor: An electric motor is a device that transforms electrical energy into mechanical energy.

The battery is linked to an electric motor as seen below:

From this, we may deduce that when the circuit is turned on, the battery will convert chemical energy to electrical energy, and an electrical motor will then transform this electrical energy into mechanical energy. It is evident from the foregoing that AC is transformed into mechanical energy by an electric motor. As a result, choice three is right.