A rectifier is used to convert _____.

A rectifier is a device that converts alternating current (AC) into direct current (DC). This conversion is essential in various applications, such as powering electronic devices that require a steady and stable DC voltage for operation. AC is characterized by its periodic reversal of direction, while DC flows in a constant direction. The rectification process involves using components like diodes, which allow current to flow in only one direction, effectively blocking the reverse current and smoothing the resulting waveform into a form closer to DC. By performing this conversion, rectifiers enable the use of AC sources, such as those from power grids, to efficiently power devices that require DC power. This fundamental operation is why the correct answer is the conversion of AC input into DC output. Other options describe different kinds of conversions or types of inputs and outputs that are not the primary function of a rectifier.