The correct answer is A. Al2O3.
Al2O3 is the most acidic of the given options because it is the most electronegative. Electronegativity is a measure of how strongly an atom attracts electrons. The more electronegative an atom is, the more likely it is to attract electrons from other atoms. This means that Al2O3 is more likely to form covalent bonds with other atoms, which are bonds in which electrons are shared between atoms. Covalent bonds are weaker than ionic bonds, which are bonds in which electrons are transferred from one atom to another. This means that Al2O3 is more likely to dissociate into ions in water, which is a characteristic of acids.
Na2O, MgO, and CaO are all basic oxides. Basic oxides are oxides that react with water to form hydroxides. Hydroxide is a base, which is a substance that can neutralize acids. This means that Na2O, MgO, and CaO are all less likely to dissociate into ions in water than Al2O3.
In conclusion, Al2O3 is the most acidic of the given options because it is the most electronegative.