Let the oxidation state of metals in given compounds be x.
(i) K2CrO4 2(+1)+x+4(−2)=0 x=8−2=+6
The oxidation state of K is K2CrO4is+6.
(ii)NbCl5 x+5(−1)=0 x=+5
Oxidation state of Nb in NbCl5 is +5.
(iii)MnO2 x+2(−2)=0 x=+4
Oxidation state of Mn is MnO2 is +4.
Thus, Cr has the highest oxidation state (+6) among the given metals.