Explanation

LASER stands for Light Amplification by Stimulated Emission of Radiation, from Einstein's 1917 concept of stimulated emission. The operative word is 'stimulated' — an incoming photon triggers an excited atom to emit a second coherent photon — which rules out the 'spontaneous emission' option. The emission is of radiation (photons), not 'rays' or 'energy' as the acronym's final word. Hence option A is the exact expansion.