Which gas is flammable and should never be combined with oxygen?

Acetylene is the correct answer as it is a highly flammable gas that can pose significant risks when combined with oxygen. The combination of acetylene and oxygen can lead to explosive reactions, especially under pressure or in confined spaces. This property is why acetylene is used in welding and cutting applications, but it must be handled with extreme caution in the presence of oxygen.

Hydrogen, while also flammable, is not the best choice in this context as it burns with a pale blue flame and is often used safely in controlled applications with oxygen when proper precautions are in place. Carbon dioxide and argon, on the other hand, are non-flammable gases and would not present a risk of combustion when combined with oxygen. The importance of understanding the specific properties of these gases is crucial in environments where medical gases are used or transported.