Oxygen and air are not synonyms. as a matter of fact air contains several gases. the most common gas in air is a) oxygen b) methane c) nitrogen d) carbon dioxide

The answer is C, nitrogen.
In normal air in the atmosphere, about 78% is nitrogen, following by oxygen, 21%
Note that when we breathe in air, the oxygen we breathed in is used in body, not nitrogen. The amount of nitrogen in unbreathed air and breathed air is about the same. Instead, we exchange oxygen with waste product, which happens to be carbon dioxide. So what we breathes our is mainly nitrogen, carbon dioxide, very little amount of oxygen and other noble gases.