The expansion of LASER is

Explanation:
LASER stands for light amplification by stimulated emission of radiation. In this, a coherent beam of light waves is generated and amplified by stimulated emission of radiation.
Stimulated emission is a type of emission in which photon emission is triggered by the outside photon. And the resulting photons which are emitted have the same phase and  are greatly amplified.

Final answer:
Hence, the correct option is D, i.e. light amplification by stimulated emission of radiation