The correct option is A -2.69 V; the reaction will not occur
Standard electrode potential of reaction [E∘] can be calculated as
Ecell=ER−EP
Where, ER=SRP of reactant, EP=SRP of product
If E∘cell=+ve, then reaction is spontaneous otherwise non - spontaneous.
Mn3+E∘1=1.51 V−−−−−−→Mn2+Mn2+E∘21.18 V−−−−−→Mn
∴ For Mn2+ disproportionation,
E∘=−1.51V−1.18V=−2.69V<0
Thus, all reactions will not occur.