Q.A LED emits visible light when its (A) junction is reverse biased (B) depletion region widens (C) holes and electrons recombine (D) junction becomes hot

[!TLDR] Forward biasing an LED causes electrons and holes to recombine at the junction, releasing the excess energy as photon …

An LED emits light through electroluminescence: when the diode is forward biased, electrons from the semiconductor's conduction band cross the junction and recombine with holes in the valence band. Each recombination event releases energy roughly equal to the material's band gap $E_g$, and in a suitably chosen direct-band-gap semiconductor, this energy is released as an emitted photon rather than as heat -- producing the LED's characteristic light output, whose colour (wavelength) is set by $E_g$. Options (A) and (B) describe REVERSE-bias behaviour (widening depletion region, no significant curren …

Recall the LED's working principle: forward bias drives electron-hole recombination at the junction, and this recombination -- not reverse break …

Confusing an LED's light-emission mechanism with a Zener diode's voltage-regulating (reverse breakdown) mechanism, since both are 'special purpose' diodes studied together in this …