A principal source of natural background radiation is ________, which is a gas from naturally-occurring uranium in the soil.

The correct answer is radon. Radon is a colorless, odorless gas that is produced from the natural decay of uranium found in soil and rocks. It can accumulate in buildings, particularly in basements or poorly ventilated spaces, leading to potential health risks. Understanding radon is crucial in the context of hazardous waste operations and emergency responses because exposure to elevated levels of radon can increase the risk of lung cancer.

In contrast, cosmic radiation originates from outer space and is not a result of soil or geological formations. Potassium-40 is a naturally occurring isotope of potassium but does not specifically relate to soil uranium decay processes. Medical radiation refers to the radiation that individuals may be exposed to during medical procedures, which is man-made and not part of the natural background radiation. Therefore, radon stands out as the primary source linked to natural background radiation associated with uranium decay.