Covalent character $ \propto \,\frac{1}{{size\,of\,cation}}$

                               $\propto $ size of anion

Among the given species order of size of cations

$N^{3+} < O^{2+} < Pb^{2+} < Ba^{2+}$

order of size of anions $O^{2-} > Cl^{-}$.

Hence the order of covalent character is 

$NCl_3 > Cl_2O > PbCl_2 > BaCl_2$

$BaCl_2$ is least covalent in nature.