Which term describes light amplification by stimulated emission of radiation?

The correct answer is "Laser," which stands for "Light Amplification by Stimulated Emission of Radiation." This term describes a process in which light is generated and amplified. The basic principle involves exciting atoms or molecules, which then emit photons. These photons can stimulate the emission of more photons, resulting in a coherent beam of light that is characteristic of lasers.

In this context, the other terms refer to related concepts but do not encapsulate the entire process of light amplification. A "photon" is simply a quantum of light; it is one of the particles involved in the laser process but does not refer to the amplification mechanism itself. "Radiant energy" is a broader term that describes energy emitted as electromagnetic radiation, encompassing various forms of light but not specifically the amplification process. "Irradiance" refers to the power of electromagnetic radiation per unit area, which measures how much light falls on a surface, but it does not describe how that light is generated or amplified.

Thus, "Laser" is the term that directly and accurately describes the concept of light amplification by stimulated emission of radiation.