Answer: CrO2Cl2
The oxidation number of manganese in MnO4− is 7.
The oxidation number of chromium in Cr(CN)63− is +3.
The oxidation number of nickel in NiF62− is +4.
The oxidation number of chromium in CrO2Cl2 is +6.
Hence, the species in which an element has oxidation number +6 is CrO2Cl2.