Which component allows current to flow in one direction but blocks it in the opposite direction?

The component that allows current to flow in one direction while blocking it in the opposite direction is a diode. Diodes are semiconductor devices that have two terminals, called the anode and cathode. In forward bias, when the anode is connected to a higher voltage than the cathode, the diode conducts electricity. However, when the voltage is reversed, the diode blocks the current, preventing it from flowing in the opposite direction.

This characteristic of diodes is crucial in various applications, such as rectifiers, which convert alternating current (AC) to direct current (DC). Other components listed, such as transistors, inductors, and capacitors, do not have this uni-directional current flow property, making the diode the correct choice for this question.