In case of:
(i) MnO4−, Mn is in the +7 oxidation state which means it has zero d (do) electrons. Hence, there is no d-d transition and no paramagnetism. Its colour is due to charge transfer spectra.
(ii) CrO42−, Cr is in the +6 oxidation state which means it has zero d (do) electrons. Hence, there is no d-d transition and no paramagnetism.
(iii) MnO42−, Mn is in the +6 oxidation state which means there is one d (d1) electron which gives rise to d-d transition and paramagnetism.
(iv) Cr2O2−7, Cr is in the +6 oxidation state which means it has zero d (do) electrons. Hence, there is no d-d transition and no paramagnetism. Its colour is due to charge transfer spectra.
Hence, option C is the correct answer.