What component allows current to flow in only one direction?

A diode is a semiconductor device that allows current to flow in only one direction, making it essential in various electronic circuits. This unidirectional flow is due to the diode's construction, which consists of a p-n junction. In simple terms, when a positive voltage is applied to the anode (p-type material), and the cathode (n-type material) is kept at a lower voltage, the diode becomes forward-biased, allowing current to pass through. Conversely, if the voltage is reversed, the diode becomes reverse-biased, preventing current from flowing. This property is crucial in rectification processes, where alternating current (AC) is converted to direct current (DC).

The other components, while essential in electronic circuits, do not serve the same unidirectional function. A transistor can control current flow but can work in both directions based on its configuration and biasing. A capacitor stores electrical energy temporarily but does not inherently control the direction of current flow. A resistor, meanwhile, is used to limit current or divide voltage but allows current flow in both directions without restriction. Thus, the unique characteristic of the diode as a one-way current device makes it the correct answer.