Which of the following substances has the greatest ionic character ?

According to Fajan's rule :
Covalent character $\propto \frac{1}{\mathrm{size} \mathrm{of} \mathrm{cation}}$
                                 $\propto \mathrm{size} \mathrm{of} \mathrm{anion}$
Among the given species order of size of cations

Order of size of anions O^2- > Cl^-.
Hence the order of covalent character is
${\mathrm{NCl}}_3>{\mathrm{Cl}}_2\mathrm{O}>{\mathrm{PbCl}}_2>{\mathrm{BaCl}}_2$
BaCl₂ is most ionic in nature.