The correct option is C Potassium permanganate is stable in acidic medium
Manganese exists in its highest stable oxidation state of +7 in potassium permanganate. Hence potassium permanganate is a very powerful oxidizing agent. Like chromium(VI) compounds, the color in this d0 ion is derived from charge transfer electron transitions. Potassium permanganate is a violet-black solid, which dissolves in water to give a deep purple solution.
In acidic solution, manganate ion is reduced to the colourless Mn2+ ion. This medium brings out the maximum ability to oxidize because of the sheer number of electrons acquired.
[MnO4]−(aq)+8H+(aq)+5e−→Mn2+(aq)+4H2O(l)
What about the reaction of potassium permanganate with a mild reducing agent like sulphite? The half reactions are:
[MnO4]−(aq)+e−→[MnO4]2−(aq)
SO2−3(aq)+2OH−→SO2−4(aq)+H2O(l)+2e−
So only option c is inappropriate.