Which of the following is a radioactive odorless gas which may require mitigation?

Asbestos is a group of fibrous minerals known for their heat resistance and is primarily a solid material rather than a gas. It is hazardous when inhaled as it can cause serious respiratory diseases, including asbestosis and mesothelioma, but it is not radioactive and does not require mitigation in the same context as gases like radon.

Formaldehyde is a colorless gas with a strong odor, commonly found in building materials and household products. While it can pose health risks, particularly at high concentrations, it is not radioactive. Therefore, it does not fit the description of a radioactive gas requiring specific mitigation like radon.

Carbon monoxide is a colorless, odorless gas produced by incomplete combustion of fossil fuels. It is highly toxic and can lead to poisoning, but it is not radioactive. Mitigation typically involves ensuring proper ventilation and functioning appliances to avoid buildup, but it does not share the radioactive property that defines radon.