## Core Concept
Helium-neon (HeNe) lasers are a type of gas laser. They are commonly used in applications such as spectroscopy, interferometry, and holography. The core concept here revolves around the classification of lasers based on their active medium.

## Why the Correct Answer is Right
The HeNe laser operates by exciting a mixture of helium and neon gases. When an electrical discharge is applied to the gas mixture, helium atoms are excited and then transfer their energy to neon atoms through collision. The neon atoms then emit light at specific wavelengths (most commonly 632.8 nm) as they return to their ground state. This process characterizes HeNe lasers as **gas lasers**, which use a gas as the lasing medium.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is blank and cannot be evaluated.
- **Option B:** This option is blank and cannot be evaluated.
- **Option D:** This option is blank and cannot be evaluated.

## Clinical Pearl / High-Yield Fact
A key fact to remember is that HeNe lasers are often used in medical and research settings for their precise and coherent light output. They have applications in ophthalmology for retinal photocoagulation and in dermatology for various skin treatments.

## Correct Answer: C.