What does “stimulated emission of radiation” mean?

Question

Gas particles in a laser emit photons of light when exposed to an electric current.
Gas particles in a laser emit coherent light when bombarded with X-rays.
Electromagnetic currents are produced by neon particles when they bump into helium particles.
Radioactive particles in a laser emit gamma rays of the same wavelength.

Answer 1

The correct answer is:

Gas particles in a laser emit coherent light when stimulated by a photon of the same energy and wavelength, a process known as stimulated emission of radiation.