Which gas is colorless, odorless, tasteless, and radioactive?

Radon is a naturally occurring gas that is colorless, odorless, tasteless, and radioactive. It is produced from the decay of uranium found in soil, rock, and water. Because it is radioactive, prolonged exposure to radon can pose health risks, particularly an increased risk of lung cancer. This characteristic of being both radioactive and imperceptible to human senses (sight, smell, and taste) makes radon a unique concern in real estate, especially in areas where it can accumulate, such as basements.

The other gases listed—carbon monoxide, methane, and natural gas—have distinct properties that do not coincide with those of radon. Carbon monoxide, for instance, is colorless and odorless but is not radioactive. Methane and natural gas, while also colorless and odorless, are primarily composed of hydrocarbons, which are not radioactive. Therefore, radon stands out as the only gas in this list that meets all the criteria specified in the question.