A(n) ___ changes AC voltage into DC voltage.

The correct choice is a converter. A converter is specifically designed to change alternating current (AC) voltage into direct current (DC) voltage. This process typically involves rectification, where the AC voltage is transformed into a unidirectional flow of current. Converters can be found in various applications, such as power supplies for electronic devices, where they not only switch AC to DC but also regulate the voltage level accordingly.

In contrast, an inverter is used to perform the opposite function, converting DC voltage back into AC voltage, which is essential for systems that need to supply AC power from a DC source, such as batteries. A transformer, on the other hand, is utilized to change the voltage level of AC power without changing its nature to DC. It operates on the principle of electromagnetic induction and is not involved in the rectification process. Lastly, a synchronous motor is a type of motor that operates in sync with the frequency of the electrical supply and does not serve the function of voltage conversion.