The correct answer is: A. Helium
Helium is an inert gas that is mixed with oxygen to give to patients suffering from restricted breathing. It is a light gas that can help to open up the airways and make it easier to breathe. Helium is also non-flammable, which makes it safe to use in medical settings.
Krypton, radon, and argon are also inert gases, but they are not typically used in medical settings. Krypton is a relatively rare gas that is used in some industrial applications. Radon is a radioactive gas that is produced by the decay of uranium. Argon is a common gas that is used in lighting and insulation.
Helium is the best choice for patients suffering from restricted breathing because it is a light gas that can help to open up the airways and make it easier to breathe. It is also non-flammable, which makes it safe to use in medical settings.