Which of the following oxides is most acidic in nature? 

All the alkaline earth metals from oxides have the molecular formula MO. These oxides are highly stable as they posses high lattice energy. All alkaline earth metal oxide are ionic compound except BeO (covalent in nature due to high polarising power of Be). Their stability decreases as the size of the cation increases. The oxides
are basic in nature, and the basic strength increases down the group in the order: BeO > MgO > CaO > SrO > BaO. Among all, BeO is comparatively more acidic (least basic), but BeO is amphoteric in nature and
dissolves in both acid and alkali. Hence, BeO is most acidic in nature than MgO, BaO, CaO.