Among the following oxides, which is least acidic?

Applying Fajans' rule, the higher the oxidation state (or charge), the more is the covalent character and the more is acidic and vice versa. Moreover, the acidic character increases from B to F.Acidic character order isNO2>CO2>B2O3>Al2O3 least acidic  or amphoteric  Period ⇒         2nd      2nd      2nd      3rd  Group ⇒       15      14       13        13