Answer:
See Explanation
Step-by-step explanation:
Let us closely consider the electronic configuration of manganese atom, Manganese II ion and manganese III ion. This will give a clue of the answer.
Manganese: [Ar] 3d5 4s2
Manganese II: [Ar] 3d5
Manganese III: [Ar] 3d4
Now, let us remember that the d orbital contains a maximum of ten electrons. Hence, the electronic configuration of Mn^2+, [Ar] 3d5 reflects a half filled d- orbital which is known to be unusually stable(half-filled orbitals are known to possess unusual stability just as fully filled orbitals do).
The same can not be said about Mn^3+. Hence, Mn^2+ is more stable than Mn^3+.