The ion having highest mobility in aqueous solution is

The correct option is D $B{a}^{2+}$
On going down the group the size of metal ion increase so hydration energy decreases. Since $B{a}^{2+}$ is the heaviest ion it is least hydrated. As a result, its size of hydrated ion becomes lesser than any other hydrated alkaline earth metal cation and hence has the highest ionic mobility.