The correct answer is option 4, Radon. Radon is a noble gas that can be used for the treatment of cancer. It emits alpha particles which are highly effective in destroying cancer cells. These particles can penetrate tissues deeply without causing significant damage to healthy surrounding tissues. Radon therapy, also known as radon inhalation therapy or radon spa therapy, involves the inhalation of radon gas in controlled doses. The radioactive properties of radon help in targeting and killing cancer cells, making it a potential treatment option for certain types of cancer. However, it is important to note that radon therapy is still a subject of ongoing research, and its use may vary depending on the specific circumstances and condition of the patient.