Explanation

(b)
We know that $Al^{3 + \ }$cation is smaller than $Na^{+}$ (because of greater nuclear charge). According to Fajan’s rule, small cation polarise anion upto greater extent. Hence, $Al^{3 + \ }$ polarise $Cl^{-}$ ions upto greater extent, therefore, $AlCl_{3}$ has covalent bond between Al and Cl atoms.