Which of the following devices or circuits changes an alternating current into a varying direct current signal?

A rectifier is a circuit (typically made using diodes) that allows current to flow in only one direction. When alternating current (AC), which reverses direction periodically, passes through a rectifier, the output becomes a varying or pulsating direct current (DC) that flows in only one direction. A transformer changes the voltage levels of an AC signal but does not convert it to DC, making it incorrect. An amplifier increases the power of a signal, and a reflector bounces radio waves; neither of these devices perform the crucial AC-to-DC conversion role of a rectifier.