Which of the following ion has maximum polarising power

According to Fajan's rules of polarization, more the size of anion, more easily will it be polarized and hence compound will be more non polar. Less the size of cation, more will be it's polarizing power and hence compound will be more non polar. Now, among all the ions given to us, the Al⁺³ ion is the smallest in size because it has lost 3 electrons so it becomes very small as compared to other ions (the effective nuclear charge increases to a maximum amount). 

Hence, Al³⁺  has maximum polarising power.