Which of the following devices or circuits changes an alternating current into a varying direct current signal?
Why A rectifier uses diodes, which conduct in only one direction, to block or flip the negative half of an AC waveform so the output current always flows the same way. The result is pulsating (varying) DC, which a filter capacitor then smooths into steady DC in a power supply. That one-way conduction is exactly what the word 'rectify' means here.
Watch out A transformer is tempting because it sits right next to the rectifier in a power supply, but it only steps AC voltage up or down through magnetic coupling; its output is still AC.
Diode = one-way street. Rectify means 'make it go one way' -> AC in, pulsating DC out.