Which component converts alternating current to direct current in a vehicle?

The component that converts alternating current (AC) to direct current (DC) in a vehicle is the alternator. The alternator generates electricity while the engine is running and produces AC as its output. However, most electrical systems in a vehicle, such as the battery and electronic components, require DC. Therefore, the alternator includes a rectifier that changes the AC produced into DC. This process ensures that the electrical system can function properly and keep the battery charged.

The other components listed have different functions within the vehicle. The battery stores electrical energy as DC and supplies it to the engine and electrical components when needed. The starter motor uses electricity from the battery to start the engine but does not convert AC to DC. The generator, while it can refer to a device that produces electrical power, in modern automotive contexts, the term typically relates more to older vehicles and may not specifically denote a component that converts AC to DC like an alternator does. Thus, the alternator is the correct choice for this function.