Which component changes electrical energy into mechanical energy to rotate the engine for startup?

The component that changes electrical energy into mechanical energy to rotate the engine for startup is the starter motor. The starter motor is specifically designed for this purpose in automotive and similar applications. When the ignition is turned on, electrical current is supplied to the starter motor, converting that electrical energy into mechanical energy. This mechanical energy then turns the engine's flywheel, allowing the engine to begin the combustion process and start running.

The generator is responsible for converting mechanical energy into electrical energy, typically supplying power to the electrical system of the vehicle or charging the battery. The transformer is used to change voltage levels in an alternating current (AC) system, but it does not perform any function related to engine startup. An alternator, while it does generate electrical power to maintain the battery's charge while the engine is running, it does not initiate the process of starting the engine by converting electrical energy to mechanical energy. Thus, the starter motor is the correct component associated with the conversion of electrical energy to mechanical energy for engine startup.