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

The correct answer is radon, which is a naturally occurring radioactive gas that is produced from the decay of uranium in soil, rock, and water. It can accumulate in homes and buildings, particularly in enclosed spaces like basements. Radon contributes significantly to natural background radiation exposure, making it a major concern for public health in many areas.

Carbon-14, although a naturally occurring isotope formed in the atmosphere from cosmic ray interactions with nitrogen, is primarily used in radiocarbon dating and does not significantly contribute to background radiation levels compared to radon. Cesium-137 and plutonium, on the other hand, are man-made isotopes associated with nuclear fallout and accidents, and they are not sources of natural background radiation. Therefore, the understanding of radon as a natural source is pivotal in recognizing the various contributors to background radiation in our environment.