Which of the following ion has maximum polarising power

 According to Fajans' rules of polarization,If the size of cation is small, more will be the polarizing power of the cation and hence the compound will be more non polar. Now, among all the ions given to us, the ${\mathrm{Al}}^{+3}$ 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). So, It will have maximum polarizing power.