Which of the following is the most basic oxide?

Bi2O3 is the most basic oxide. The more the oxidation state of the central metal atom, the more its acidity. Hence, SeO2 (O.S of Se = +4) is acidic. Further, for a given oxidation state, the basic character of the oxides increases with increasing size of the central atom. Thus, Al2O3 and Sb2O3 are amphoteric and Bi2O3 is basic.