1. Identify the key feature of the given $I$-$V$ graph

   The graph shows that:

   • In one biasing direction, the current remains very small for a while.
   • After a certain reverse voltage, the current increases sharply while the voltage remains nearly constant.

   This is the characteristic feature of reverse breakdown.

2. Compare with known device characteristics

   (A) Solar cell

   A solar cell has an $I$-$V$ curve shifted due to photo-generated emf, and it operates in the fourth quadrant under illumination. This does not match the given curve.

   (B) Transistor used as an amplifier

   A transistor does not have a simple two-terminal $I$-$V$ characteristic like the one shown. Its output/input characteristics are different and depend on three terminals. So this is not correct.

   (C) Diode used as a rectifier

   A rectifier diode conducts strongly in forward bias, but in reverse bias it ideally has only a small leakage current and is not intended to operate in breakdown. The graph with sharp reverse breakdown is not the usual rectifier diode characteristic.

   (D) Zener diode used as a voltage regulator

   A zener diode is specially designed to operate in reverse breakdown region, where a large current flows with nearly constant voltage. This matches the graph exactly.

3. Conclusion

   Therefore, the device is a zener diode, which is used as a voltage regulator.

$\boxed{\text{Option D}}$