Concept:A rectifier is an electronic device that converts alternating current (AC) into direct current (DC).Explanation:Alternating current continuously reverses its direction of flow.Direct current flows steadily in only one direction.A rectifier uses semiconductor diodes to permit current flow in one direction only.It blocks the opposite half of the AC cycle, producing a one-way current.This output is a pulsating DC that can be smoothed using a filter capacitor.Therefore, the main function of a rectifier is to change AC into DC.Answer:D. A.C to D.C