The order of which of the following oxides is arranged according to decreasing basic nature?

Applying Fajans' rule, the lower the charge; the more is ionic and the more is basic and vice versa.Moreover, small cation, large anion, high charge and presence of d electrons mean more covalent and more acidic.Therefore, decreasing order of basic nature isSize Al3+ > Cu2+ and presence of d electron in Cu2+ makes it less basic (or more acidic) than Al2O3.