In inorganic chemistry, Fajans' rules, formulated by Kazimierz Fajans in 1923, are used to predict whether a chemical bond will be covalent or ionic, and depend on the charge on the cation and the relative sizes of the cation and anion. They can be summarised in the following table:

$\begin{array}{|l|l|} \hline \text{Ionic } & \text{Covalent } \\ \hline \text{Low positive charge on cation} & \text{High positive charge on cation} \\ \hline \text{Large cation} & \text{Small cation} \\ \hline \text{Small anion} & \text{Large anion} \\ \hline \end{array}$

In the given case all the anions are same in all the given compounds. So we need to think about cations.

The ionic radius of ${\mathrm{Ag}}^{+}$ is less than that of ${\mathrm{Cl}}^{-}$ but their charges are equal. Hence, $\mathrm{AgCl}$ is a least ionic compound among the given options.

$\mathrm{AgCl}<{\mathrm{CoCl}}_2<{\mathrm{BaCl}}_2<\mathrm{KCl}$ (ionic character)