The correct answer is oxygen.
Oxygen plays a role of gas in the rusting of iron and respiration.
Oxygen is termed as a chemical element with atomic number eight and symbol O. It is a member of the chalcogen group.
It is a reactive nonmetal. It is an oxidizing agent which forms oxides.
Dioxygen is used in cellular respiration and in organic molecules which is in living organisms which contains oxygen. For example, fats, nucleic acids, and carbohydrates.