What is a common source of natural radioactive exposure?

Radon gas from the earth is a common source of natural radioactive exposure because it is a naturally occurring radioactive gas that is released from the decay of uranium found in soil, rock, and water. Radon can accumulate in homes, especially in basements and poorly ventilated areas, leading to increased exposure for occupants. It is recognized as a significant health risk, particularly due to its association with lung cancer when inhaled over long periods.

The other options do not represent natural sources of radiation. Artificially created nuclear reactors are human-made and not part of the natural environment. Medical X-ray procedures also involve artificial sources of radiation used in medical diagnostics and treatment. Non-radioactive minerals, by definition, do not emit radiation and hence cannot contribute to radioactive exposure.