Which one among the following oxides has the highest melting point?

MgO (magnesium oxide) has the highest melting point among the given oxides, at approximately 2852°C. This is due to its strong ionic bonding with high lattice energy resulting from the small size and high charge density of Mg2+ and O2- ions (both doubly charged). Na2O melts around 1132°C, Fe2O3 around 1565°C, and CuO around 1326°C. MgO's exceptionally high melting point makes it useful as a refractory material in furnace linings.