Which of the following is NOT a geological source of Radon?

Marble is not considered a significant geological source of radon compared to other materials listed. Granite, shale, and certain types of soil are known to contain uranium, which decays to produce radon gas. This process occurs in rocks and soil that have higher concentrations of uranium, which can lead to higher potential radon emissions.

Granite, for example, is a common source of radon due to its generally high uranium content. Shale, on the other hand, is also known to release radon gas because it often contains organic materials that can be a source of uranium. Soil can accumulate radon as it is produced from the breakdown of uranium-rich materials underground.

In contrast, marble is generally formed from limestone and does not typically contain the concentrations of uranium or radium necessary to produce noticeable levels of radon. Hence, it is not recognized as a geological source of radon, making it the correct choice for this question.