Which of the following ion has maximum polarizing power?

In chemistry, certain ions will show polarizing power this is due to the ratio of charge to radius of that ion.

$\mathrm{Polarizing} \mathrm{power}=\frac{\mathrm{Ionic} \mathrm{charge}}{{(\mathrm{Ionic} \mathrm{radius})}^2}$

From the above formula we should know that the ion which has greater ionic charge and smaller ionic radius that has high polarizing power.

Among the given all ions ${\mathrm{Al}}^{+3}$has maximum polarizing power than other ions. This is because,  ${\mathrm{Al}}^{+3}$has greater ionic charge (+3) and smaller ionic radius.