Which of the following is NOT a non-flammable medical gas?

Acetylene is classified as a flammable gas, which makes it distinct from the other options listed as non-flammable medical gases. Oxygen, while it supports combustion, is not considered flammable on its own but is essential for combustion to occur. Nitrous oxide, used commonly in medical settings, acts as an anesthetic and is also considered non-flammable. Medical air, which is a mixture of oxygen and nitrogen, is used for various therapeutic purposes and is likewise classified as non-flammable. Thus, acetylene stands out as the only gas among the choices that is inherently flammable, confirming it as the correct answer to the question regarding which is NOT a non-flammable medical gas.