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

Radon is considered a significant source of natural background radiation. It is a radioactive gas that is produced from the decay of uranium found in soil, rock, and water. As radon disperses from the ground, it can accumulate in buildings, particularly in poorly ventilated areas. It contributes to the natural radiation exposure that people receive, and in some regions, it can be a major contributor to overall radiation dose.

Other options, such as medical X-rays, nuclear reactors, and dental X-rays, are sources of artificial or man-made radiation rather than natural background radiation. While these sources can significantly increase individual exposure to radiation, they do not fall under the category of natural background radiation because they result from human activities or medical interventions rather than naturally occurring processes.