Which one of the following alkaline earth metal ions has the highest ionic mobility in its aqueous solution?

Ionic mobility in aqueous solution depends on the hydrated ionic radius, with a smaller hydrated radius corresponding to higher ionic mobility. Among alkaline earth metal ions, the smaller bare ions (like $$Be^{2+}$$ and $$Mg^{2+}$$) have higher charge density, attracting more water molecules and thus forming a larger hydrated radius. In contrast, larger bare ions (like $$Sr^{2+}$$) have lower charge density, attract fewer water molecules, and therefore exhibit a smaller hydrated radius.

The order of hydrated ionic radii is $$Be^{2+} > Mg^{2+} > Ca^{2+} > Sr^{2+}$$. Since ionic mobility is inversely related to the hydrated ionic radius, the mobility order becomes $$\text{Ionic mobility: } Sr^{2+} > Ca^{2+} > Mg^{2+} > Be^{2+}$$. Hence, $$Sr^{2+}$$ has the highest ionic mobility in aqueous solution due to its smallest hydrated radius among the given options. The correct answer is Option A.