Explanation: Light-emitting diodes (LEDs) are current-driven devices that will draw excessive current and quickly burn out if connected directly to a voltage source greater than their forward voltage. A series resistor protects the LED by limiting the current flowing through it. The resistor drops the excess voltage, allowing the LED to operate at its specified forward current, which is critical for its longevity and proper operation in any electronic circuit, including amateur radio projects. A bypass capacitor to ground (A) is used to filter noise or stabilize power supply rails, not to limit DC current. An electrolytic capacitor (B) is primarily for filtering or energy storage and would block DC current to the LED. A shunt coil in series (D) is a contradictory term; an inductor in series would offer very low DC resistance and thus no current limiting for the LED.