The correct answer is C: Diode. A diode is the electronic component that allows current to flow in only one direction. Diodes are semiconductor devices with a PN junction that conducts in one direction (forward) and blocks in the other (reverse). Diodes are fundamental semiconductor devices that act as one-way valves for current. In the forward direction (positive voltage on anode, negative on cathode), they conduct current. In the reverse direction, they block current (until breakdown voltage). This unidirectional property makes diodes essential for rectification (converting AC to DC), protection circuits, signal routing, and many other applications. Understanding diodes is critical for all electronics work, especially in power supplies and RF circuits.