Assertion  :   ${\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$  is a stronger acid than ${\left[\mathrm{Mg}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$

Reason    :     Size of ${\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$ is smaller than  ${\left[\mathrm{Mg}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$ and has a higher effective nuclear charge. 

Oxidation state of Al in ${\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_4\right]}^{3+}$ is +3 whereas the oxidation state of Mg in ${\left[\mathrm{Mg}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$ is +2. Size of Al³⁺ is smaller than Mg²⁺ due to high magnitude of positive charge.  Also Al³⁺ possesses more effective nuclear charge and attracts electron pairs from donor ligand more effectively than Mg²⁺. This gives rise strong acidic nature of ${\left[\mathrm{Al}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$ than ${\left[\mathrm{Mg}\left({\mathrm{H}}_2\mathrm{O}\right)\right]}^{2+}.$