Consider following oxides : $\mathrm{BaO},\mathrm{CaO},{\mathrm{Rb}}_2\mathrm{O},{\mathrm{Cs}}_2\mathrm{O}$ and select correct option(s).
 

By the rule of Fajan, covalent character increases with increase in size of the anion. Thus,

Most covalent $\mathrm{CaO}$, least covalent ${\mathrm{Cs}}_2\mathrm{O}$. Also,

Most ionic = ${\mathrm{Cs}}_2\mathrm{O}$, least ionic $\mathrm{CaO}$

Thus, the correct option is B and C.