Ba<Ca<Se<S<Ar
Ba and Ca belong to group IIA ( alkaline earth metals).
Se and S belong to group Vl A (Chalcogens) and as we go down the group ionization enthalpy decreases with increasing atomic size. So,
Ba<Caand Se<S
Ar has stable completely filled electronic configuration and, therefore, has the highest ionization enthalpy.Hence, the correct order is
inkJmol−1Ba503<Ca590<Se941<S999<Ar1520