For a light-emitting diode (LED) to emit light, it must be used in a forward-biased condition. Therefore, the correct answer is: Option D : always used in forward biased condition When an LED is forward biased, the electrons in the n-type material have enough energy to cross the p-n junction and recombine with holes in the p-type material. This recombination process results in the release of energy in the form of photons, which is the light that we see emitted from the LED. On the other hand, when an LED is in reverse bias, it does not emit light because the potential barrier at the p-n junction is increased, preventing electron-hole recombination.