Which of the following is NOT a place where hydrogen may be found in underground coal mines?

Hydrogen is a gas that can be generated in various environments, including underground coal mines. It is crucial to understand the contexts in which hydrogen can occur. Among the options provided, stagnant water is typically not a significant source or location for hydrogen within underground coal mining contexts.

In battery charging stations, hydrogen can be released due to the electrolysis of water, especially if there are malfunctions during charging processes that involve water systems. In heating systems, particularly those used in underground mines, there may be combustion processes or other reactions that can produce or release hydrogen gas.

The return side of an active fire in a coal mine can also produce hydrogen, as fires generate various gases as a result of combustion. In contrast, stagnant water does not typically result in hydrogen production under normal conditions since it lacks the necessary energy or processes (such as electrolysis or high temperatures from combustion) to release hydrogen gas.

Therefore, selecting stagnant water as the option where hydrogen is NOT likely found in underground coal mines is accurate, as it is not a common source or environment for the generation of hydrogen in that setting.