The correct option is A 1.70V
The relationship between standard Gibbs energy change for a reaction and its cell potential is ΔGo=−nFEocell.
For manganese electrodes, ΔGoMnO−4|Mn2+=ΔGoMnO4−|MnO2+ΔGoMnO2|Mn2+−nFEoMnO−4|Mn2+=−nFEoMnO−4|MnO2−nFEoMnO2|Mn2+5EoMnO−4|Mn2+=3×EoMnO−4|MnO2+2×EoMnO2|Mn2+
Substitute values in the above equation,
5×1.51V=3×EoMnO−4|MnO2+2×1.23VEoMnO−4|MnO2=1.70 V