Which are three geological sources of radon?

The correct choice highlights three geological sources of radon: granite, shale, and phosphate. These materials are particularly significant because they contain uranium and thorium, which are naturally occurring radioactive elements. When uranium and thorium decay, they produce radon gas as a byproduct.

Granite, for example, is an igneous rock that can contain substantial amounts of uranium. Shale, a sedimentary rock, also has the potential to trap radon due to its fine-grained nature and ability to retain gases. Phosphate deposits, often associated with sedimentary environments, can contain uranium as well and thus contribute to radon production.

In contrast, the other choices include materials that are less associated with significant radon production, primarily due to their lower uranium content or geological characteristics that are not conducive to radon release. For effective radon mitigation and understanding, recognizing these specific geological sources is essential, which is well captured in the correct choice.