Which of the following is NOT an active medium of lasers?

The correct answer is plasma, as it is not considered a traditional active medium for lasers. Lasers operate on the principle of stimulated emission of radiation, and the active medium is the material that amplifies light. In most laser systems, active media include gas, solid, and liquid forms, each of which can be excited to produce photons that contribute to laser action.

Gas lasers, like helium-neon or carbon dioxide lasers, utilize gases as their medium. Solid-state lasers employ solid materials like ruby or Nd:YAG, where the active ions provide the gain. Liquid lasers, such as dye lasers, use organic dye solutions as their medium.

Plasma, while it can produce light and may be involved in certain light-emitting processes, is not typically categorized as an active medium in laser technology. Instead, it's more associated with phenomena like fusion or as an ionized medium that might support other types of processes. Thus, plasma does not fit the definition of an active medium in the context of laser operation.