Which part turns AC power into DC power?

The correct choice in this context is the alternator. An alternator generates alternating current (AC) and is a crucial component in automotive electrical systems. Although its primary function is to provide AC power, the alternator is typically coupled with a rectifier, which converts this AC power into direct current (DC).

This transformation is essential because most automotive systems, like the battery and electronic devices, operate on DC power. Therefore, the specific part that facilitates the conversion of AC power to DC power in vehicles is the rectifier associated with the alternator, but the alternator is often the component referred to when discussing sources of electrical power generation in this context.

The other options do not fit this function as accurately. A transmitter typically relates to the transmission of signals rather than power conversion, and a generator often refers to devices that primarily produce DC power directly, which does not pertain to converting AC to DC. An inverter, on the other hand, is used to convert DC power back into AC, rather than the other way around. Thus, the alternator, in its function of working with a rectifier, is directly responsible for turning AC power into DC power for use in motor vehicles.