Which of the following is considered a primary natural source of background radiation?

Radon gas is recognized as a primary natural source of background radiation due to its presence in the environment as a decay product of uranium and thorium found in soil and rocks. It is colorless, odorless, and can accumulate in buildings, particularly in poorly ventilated areas. The decay of radon releases alpha particles, contributing significantly to the natural radiation exposure that individuals receive over time. This exposure is considered a significant portion of the total background radiation in many regions. Understanding the impact of radon gas helps in assessing radiation exposure levels in homes and developing effective safety measures to mitigate risks associated with natural radiation sources.

In contrast, options like medical imaging, nuclear power plants, and industrial applications are not considered primary natural sources of background radiation. Medical imaging involves artificial sources of radiation generated for diagnostic purposes, while nuclear power plants and industrial applications also involve human-made sources of radiation, which do not contribute to natural background radiation levels.