`Mg^(2+)` ion more highly hydrated than `Na^(+)` ion.

Explanation

The positive charge density around `Mg^(2)` ion is more than around `na^(+)` ion due to its smaller size and more number positive chrges. Therefore,`Mg^(2+)` ion has a greater tendency to attract surrounding `H_(2)O` molecules from water than `Na^(+)` ion. In other words, it is more highly hydrated than `Na^(+)` ion.