Explanation

(a)
According to Fajan’s rule, as the charge on cation increase its size decreases. As a result its tendency to polarise anion increases. This brings more and more covalent character to electrovalent compounds.
$\because\ \$Among$\ AlCl_{3},LaCl_{3},\ MgCl_{2}\$and$\ CsCl\$size of$\ Al^{3 +}\$is smallest$.$
$\therefore\ Al^{3 +}$ polarises anion to highest extent.
$\therefore\ AlCl_{3}\$has maximum covalent character.