**Core Concept**
Helium is a noble gas with unique physical and chemical properties. Its low atomic mass and high thermal conductivity make it useful for various medical and industrial applications.

**Why the Correct Answer is Right**
The correct answer is due to helium's properties as an inert gas, which means it does not readily react with other elements. This property is a result of its full outer energy level, making it chemically stable. Additionally, helium's low density and high thermal conductivity make it an excellent choice for medical applications, such as respiratory therapy and MRI cooling systems.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because helium is not a diatomic molecule, unlike nitrogen and oxygen.
**Option B:** This option is incorrect because helium is not a toxic gas, unlike carbon monoxide and hydrogen cyanide.
**Option C:** This option is incorrect because helium is not a radioactive gas, unlike radon and radon-222.

**Clinical Pearl / High-Yield Fact**
Helium is used as a carrier gas for anesthesia due to its low viscosity and high diffusion rate, which allows for faster induction and recovery times.

**Correct Answer: C.**