The correct options are
A Mn2+/MnO−4
B Cr3+/CrO2−4
D NO2/NO−3
EOP will vary with the pH of solution for Mn2+/MnO−4,Cr3+/CrO2−4andNO2/NO−3. This is because in the net electrode reaction, the concentrations of various species depends on the hydrogen in concentration.
The pH will not vary for the Ni/Ni2+. This is because in the net electrode reaction, the concentrations of various species does not depends on the hydrogen in concentration.