Which of the following environments is most likely to have high radon levels?

Mountainous regions are most likely to have high radon levels due to the geological characteristics commonly found in these areas. Radon is a naturally occurring radioactive gas that is produced from the decay of uranium found in soil, rock, and water.

In mountainous regions, the presence of granite and other uranium-rich rocks contributes significantly to radon production. The elevation can also influence the accumulation of radon in homes, especially in poorly ventilated structures built in these areas. As radon gas seeps through cracks and openings in the Earth's surface, it can become concentrated in enclosed spaces, particularly basements.

Contrarily, coastal areas typically have more sandy or softer sediment with lower uranium content, which results in lower radon levels. Desert environments may have variations depending on the geology but generally do not have the same elevated levels of radon as mountainous areas. Low-lying plains, similar to coastal areas, often consist of sedimentary deposits that are less likely to contain significant amounts of uranium, leading to reduced radon levels compared to mountainous regions.

Thus, the geological complexity and rock types found in mountainous areas play a crucial role in often resulting in higher radon concentrations compared to other environments.