Amongst the following, identify the species which has an oxidation number of +6 with an atom?

MnO4- Cr2O72- [Cr(NH3)6]Cl3 K4[Fe(CN)6]

Cr2O72-

In Cr2O72-  2(x) + 7(-2) = -2 2x - 14 = -2 2x = -2 + 14 2x = 12 x = +6