The correct order of polarizing ability of cations of Alkaline earth metals is

Ba²⁺ < Ca²⁺ < Mg²⁺ < Be²⁺

As the size of cation decreases nuclear charge increases, and hence distortion in the electron cloud of anion by the cation increases, hence polarizing power increases.
Polarising power of cation
The ability of a cation to cause distortion in the electron cloud of anion is called polarizing power .
As the polarizing power of cation increases sharing of electron between bonded atoms increases and hence covalent character increases.