Which of the following devices will allow current to flow in ONLY one direction?

A diode is a semiconductor device that permits current to flow in only one direction, making it essential for controlling electrical signals in various applications. Its unique property is due to the arrangement of its p-n junction, which allows electrons to move in one direction while blocking movement in the opposite direction. This characteristic is crucial in circuit design for rectification processes, where alternating current (AC) needs to be converted into direct current (DC).

The other options do not possess this unidirectional flow capability. Primary current refers to the initial current supplied from a power source, while a step-down transformer is used to convert voltage levels and can allow current to flow in both directions. A range tap is associated with adjusting or accessing different voltage levels in circuits but does not specifically control current direction. Thus, the diode stands out for its fundamental function in ensuring that current can only move in one specified direction.