Answer the following question:
The relative stability of the oxidation states can be analyzed from the electrode potentials. Lower the reduction potential i.e. higher the negative value for M2+|M, higher will be the stability of the +2 oxidation state.
For 3d3 4s2 i.e. Vanadium, Eo = -1.18 so high negative value indicates that +2 state is more stable since it gives half filled t2g3 stable configuration.
For 3d5 4s2 i.e. Manganese, Eo = -1.18 which means that +2 state is more stable since it gives the stable half filled d5 configuration.
For 3d6 4s2 i.e. Iron, Eo = -0.44 so low negative value indicates that +2 is less stable for iron since it can easily lose one more of its electron to attain the half filled stable d5 configuration.