Which device allows current to flow primarily in one direction?

• A. Diode
• B. Battery
• C. Light bulb
• D. Resistor

Answer: (A)

This question focuses on a component that directs current to flow mainly in one direction. A diode is designed exactly for that behavior: it conducts readily when it’s forward biased (anode more positive than the cathode) and blocks when it’s reverse biased (cathode more positive). In forward bias, charge carriers cross the junction and a small voltage drop remains, often about 0.6–0.7 volts for silicon diodes. In reverse bias, the depletion region at the junction widens and current is greatly limited, so virtually no current flows except for a tiny leakage.

The other options don’t enforce one-way flow. A battery provides energy to push current through a circuit but doesn’t restrict direction on its own. A resistor or a light bulb conducts current in either direction according to the circuit, without a built-in mechanism to block the opposite direction.

So, the component that allows current to flow primarily in one direction is the diode.