Explanation

(d)
$Covalent\ character\ \propto \frac{1}{size\ of\ cation}$
$\propto size\ of\ anion$
(according to Fajan’s rule)
Lower the covalent character, higher will be ionic character.
${Cl}_{2}O,\ contains\ O^{2 -},\ NCl_{3}\ contains\ N^{3 -}$
$,\ PbCl_{2}\ contains\ Pb^{2 +}and\ BaCl_{2}\ contains\ Ba^{2 +}.$
Hence, the order of covalent character is
$NCl_{3} > Cl_{2}O > PbCl_{2} > BaCl_{2}$
$\therefore BaCl_{2}$ has the greatest ionic character.