Answer
Please see the work below.
Work Step by Step
Given that the binding capacity of alkaline earth metal ions towards phosphate anion is as follows, $Ba^{2+} \lt Sr^{2+} \lt Ca^{2+} \lt Mg^{2+}$.
This is due to the largest charge density of $Mg^{2+}$ ion. $ Mg^{2+}$ ion is the smallest among the given ions as a result its charge density is the largest and it effectively attracts the anion towards it.
Similarly in the given ions, $ Ba^{2+}$ is the biggest as a result its charge density is least among the given ions and it attracts the anion less effectively towards it.
