Let oxidation state of Fe is 'x' in the given compounds. (a) K2[FeO4]:+2+x+4(−2)=0 x=+6 (b) FeSO4.(NH4)2SO4.6H2O x+2(−2)+2(1)+6(0)=0 x=+2 (c) [Fe(CN)6]3− x+6(−1)=−3 X=+3 (d) K2[Fe(OH)6] 2(1)+x+6(−1)=0 X=+4 Hence, least oxidation state of Fe is in compound FeSO4.(NH4)2SO4.6H2O