If air is taken as a binary solution, the solvent is

Binary solution is a solution of two substances or liquids which are completely miscible with each other. Boiling point of binary solution depends upon the solution composition.
Now if we consider volume, dry air contains 78.08% of nitrogen, 20.95% of Oxygen, 0.93% of Argon, 0.04% carbon and other trace gases.
Since, Nitrogen is present in more quantity as compared to other gases, it is the solvent.