We need the species that is stable in aqueous solution.

Check each option using known redox behavior of transition-metal ions in water.

Option A: $Cr^{2+}$

$Cr^{2+}$ is a strong reducing agent in aqueous solution and is easily oxidized to $Cr^{3+}$ by dissolved oxygen.

$4Cr^{2+} + O_2 + 4H^+ \rightarrow 4Cr^{3+} + 2H_2O$

So, $Cr^{2+}$ is not stable in aqueous solution.

Option B: $Cu^+$

In aqueous solution, $Cu^+$ undergoes disproportionation:

$2Cu^+ \rightarrow Cu^{2+} + Cu$

Hence $Cu^+$ is unstable in water.

Option C: $MnO_4^{3-}$

This is hypomanganate(V). It is highly unstable in aqueous solution and disproportionates/changes to other manganese oxidation states.

Therefore, $MnO_4^{3-}$ is not stable in aqueous solution.

Option D: $MnO_4^{2-}$

This is manganate(VI). It is relatively stable only in strongly alkaline medium, but among the given species it is the one known to exist in aqueous alkaline solution.

In standard inorganic chemistry/JEE context, $MnO_4^{2-}$ is considered stable in aqueous solution (alkaline medium) compared with the others, which are definitely unstable.

Conclusion

The stable species is:

$\boxed{MnO_4^{2-}}$

So the correct option is D.