We know that CO is a neutral ligand and K carries a charge of +1. Therefore, the complex can be written as K+ [Co(CO)4]?. Therefore, the oxidation number of Co in the given complex is ?1. Hence, option (iii) is correct.
The central metal ion in all the three complexes is the same. Therefore, absorption in the visible region depends on the ligands. The order in which the CFSE values of the ligands increases in the spectrochemical series is as follows: H2O < NH3 < NO2? Thus, the amount of crystal-field splitting observed will be in the following order:?0 (H2O) < ?0(NH3) < ?0(NO2? Hence, the wavelengths of absorption in the visible region will be in the order:[Ni(H2O) 6]2+ > [Ni(NH3)6]2+ > [Ni(NO2)66]4?.