Which statement best describes a diode's primary function?

• A. Allows current to flow in one direction
• B. Allows current to flow in both directions
• C. Stores electrical energy
• D. Blocks all current

Answer: (A)

A diode is designed to steer current so it flows predominantly in one direction. When the anode is more positive than the cathode (forward bias), the diode conducts and current can pass through. When the polarity is reversed (reverse bias), it resists current flow and blocks it, protecting circuits from reverse currents. This unidirectional behavior is the defining feature and is what makes diodes essential for rectification in power supplies and for isolating signals.

It’s not about storing energy—that’s the role of capacitors or inductors—nor is it about blocking all current, since a forward-biased diode does conduct, albeit with a small voltage drop. It also isn’t about allowing current in both directions, which would be the behavior of a resistor or a bilateral device.