(A) In the reaction : O2−​⟶O2​+O22−​Oxygen on reactant side is in −1/2 oxidation state. In product side, one of the oxygen is in zero oxidation state, i.e. oxidised while the other oxygen is in -1 oxidation state, i.e. reduced. Hence, in the above reaction, oxygen (O−1/2) is simultaneously oxidised and reduced disproportionated.(B) In acidic medium, CrO42−​ is converted into Cr2​O72−​ which is a dimeric, bridged tetrahedral.(C) MnO4−​+NO2−​+H+⟶Mn2++NO3−​The above is a redox reaction and a product NO3−​has trigonal planar structure.(D) NO3−​+H2​SO4​+Fe2+⟶Fe++NOThe above is a redox reaction.