Max. Covalency

Fajan's Rules state that

1) The polarising power of the cation increases with increase in magnitude of +ve charge on the cation.

2) The polarising power of cation increases with decrease in size of cation

Thus , Polarising power is directly proportional to charge of cation and inversely proportional to size of cation.

But extent of polarisation is directly proportional to covalent character in an ionic bond.

Of the following options given $Aluminium$ is the only cation satisfying both the rules.

$Al^{3+}$ has the highest charge.

$Al^{3+}$ has the smallest size

Thus , $Al^{3+}$ provides best polarisation of the electron cloud of $Cl^{-}$ anion.

Hence , $Aluminium~Chloride$ has the highest covalent character of the given compounds.