Which component allows current to flow in only one direction?

The correct answer is the component that permits current to flow in only one direction, which is a diode. A diode is a semiconductor device that has high resistance in one direction and low resistance in the other, effectively allowing it to act as a one-way valve for electrical current. This property is crucial in various applications, such as rectification, where alternating current (AC) is converted into direct current (DC).

In circuits, diodes are used to prevent reverse current that could potentially damage sensitive components. Their operation is based on the semiconductor material's characteristics, often silicon, which creates a junction that controls the flow of electrons.

Other components listed do not share this particular directional flow characteristic. For instance, a transistor is a semiconductor device that can amplify or switch electronic signals but does not inherently restrict current flow to a single direction. A variable resistor allows for the adjustment of resistance in a circuit and can accommodate current in both directions depending on the configuration. Lastly, a capacitor stores and releases electrical energy but does not control the direction of current flow, functioning instead by charging and discharging in response to the circuit conditions.