Which part is primarily responsible for starting the engine?

The starter motor is the component primarily responsible for starting the engine in a vehicle. When you turn the ignition key or push the start button, electrical current flows to the starter motor, which engages the engine's flywheel. This action causes the engine to turn over and initiate the combustion cycle, ultimately allowing the engine to start running.

Each of the other components listed serves different functions critical to the operation of a vehicle. The fuel pump delivers gasoline to the engine, which is necessary for it to run but does not actually start the engine. The timing belt regulates the timing of the engine's valves and pistons, ensuring proper synchronization during operation, but it does not directly engage to start the engine. The transmission, on the other hand, allows the engine's power to be transferred to the wheels when the vehicle is in motion; it is not involved in starting the engine itself. Therefore, the starter motor is clearly defined as the part that initiates the engine's operation.