What does a diode do in an electrical circuit?

A diode is a semiconductor device that serves a specific function in an electrical circuit: it allows current to flow in one direction only. This directional flow is achieved through the diode's construction, which comprises a p-n junction. When the voltage is applied in the correct direction (forward bias), the diode becomes conductive, allowing electric current to pass through. Conversely, when the voltage is applied in the reverse direction (reverse bias), the diode blocks current flow, effectively preventing it from traveling in that direction.

This unique property of diodes is essential in various applications, including rectification (converting alternating current to direct current), protecting circuits from reverse voltage, and enabling specific signal processing tasks. The ability to control the direction of current is vital for ensuring proper operation within different electronic devices and circuits. Overall, this is why the correct answer identifies the function of a diode as allowing current to flow in one direction only.