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:
Ionic Low positive charge on cationLarge cationSmall anionamp;Covalent amp;High positive charge on cationamp;Small cationamp;Large anion
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 Ag+ is less than that of Cl− but their charges are equal. Hence, AgCl is a least ionic compound among the given options.
AgCl<CoCl2<BaCl2<KCl (ionic character)