The basicity of an oxide is closely related to the electropositive character of the element. The more electropositive the element, the more basic its oxide will be. This is because the oxide ion can react with a cation to form a basic oxide. As we move down Group 13, the electropositive character of the elements increases, and so does the basicity of their oxides.
B2O3<Al2O3<Ga2O3<In2O3<Tl2O3
So most basic oxide among the given options is Tl2O3.