Explanation: Most common light-emitting diodes (LEDs) used as indicators or in basic circuits, like those found in amateur radio projects, are designed to operate optimally around 20 milliamperes (mA). This current provides good brightness and efficiency without risking damage to the component. Lower currents like 5 mA or 10 mA would make the LED glow, but typically dimmer than intended for general indication. A current of 400 mA, however, is significantly too high for a standard indicator LED and would quickly destroy it. While high-power LEDs exist that operate at much higher currents, the question refers to a general light-emitting diode, for which 20 mA is the widely accepted approximate operating current.