Which device or circuit changes an alternating current into a varying direct current signal?

A rectifier is the device that converts alternating current (AC) into varying direct current (DC). This conversion is essential in many applications, especially in electronics, where DC is needed to power devices or to provide a stable voltage output. A rectifier achieves this by allowing current to flow in one direction while blocking it in the opposite direction. This is typically done using diodes, which only conduct electricity one way. As a result, the output of a rectifier typically has a pulsating DC voltage, which can then be smoothed out further with additional filtering components if a more stable DC is required.

Transformers are used to change the voltage level of AC, but they do not convert AC to DC. An amplifier increases the strength of a signal but does not change its form from AC to DC. A reflector is unrelated to these processes and typically pertains to the reflection of signals, such as in antennas. Hence, the rectifier is specifically designed for the task of changing AC into varying DC, making it the correct answer.