In which species is the named transition metal in its highest possible oxidation state?
manganese in MnO2\text{MnO}_2MnO2
vanadium in [VO2]+[\text{VO}_2]^+[VO2]+
chromium in Cr2O3\text{Cr}_2\text{O}_3Cr2O3
copper in Cu2O\text{Cu}_2\text{O}Cu2O