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

A rectifier is an electrical circuit component, usually consisting of one or more diodes, that allows current to flow in only one direction. This property enables it to take alternating current (AC), which reverses direction periodically, and convert it into a varying direct current (DC) that pulses but flows only one way. Other options serve entirely different purposes in a circuit. Transformers step voltage up or down but their output remains AC, amplifiers increase the power of a signal, and reflectors are physical elements of a directional antenna system rather than power supply components.