Which of the following would best protect a LED from reverse voltage?

A diode is the best choice for protecting an LED from reverse voltage because it allows current to flow in only one direction. When used in parallel with the LED, the diode will conduct if the voltage across the LED becomes negative (reverse polarity), effectively shunting any reverse current away from the LED and preventing damage. This is crucial because LEDs are sensitive components that can easily be damaged if subjected to reverse voltage beyond their specified rating.

In contrast, a capacitor stores electrical energy and does not inherently provide protection against reverse voltage; it can even exacerbate problems by holding reverse charge that could harm the LED. A resistor, while it can limit current, does not block reverse voltage, and therefore wouldn’t protect the LED effectively. Lastly, a fuse is designed to break the circuit when excessive current flows, but it provides no protection from reverse voltage during normal operation. Thus, the diode's specific function to clamp reverse voltage makes it the optimal choice for this application.