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$\text{Hint: Chromium's maximum oxidation state is } +6\text{.}$ $\text{Let's calculate the oxidation state of the central atom in each species:}$ $1.\ \text{MnO}_4^{-}$ $\text{Let the oxidation state of Mn be } x\text{.}$ $\text{Oxygen typically has an oxidation state of } -2\text{.}$ $x + 4 \times (-2) = -1$ $x - 8 = -1$ $x = +7$ $\text{So, Mn is in the } +7 \text{ oxidation state.}$ $2.\ \text{Cr}(\text{CN})_6^{3-}$ $\text{Let the oxidation state of Cr be } x\text{.}$ $\text{The cyanide ligand (CN) has a charge of } -1\text{.}$ $x + 6 \times (-1) = -3$ $x - 6 = -3$ $x = +3$ $\text{So, Cr is in the } +3 \text{ oxidation state.}$ $3.\ \text{NiF}_6^{2-}$ $\text{Let the oxidation state of Ni be } x\text{.}$ $\text{Fluorine always has an oxidation state of } -1\text{.}$ $x + 6 \times (-1) = -2$ $x - 6 = -2$ $x = +4$ $\text{So, Ni is in the } +4 \text{ oxidation state.}$ $4.\ \text{CrO}_2\text{Cl}_2 \text{ (Chromyl chloride)}$ $\text{Let the oxidation state of Cr be } x\text{.}$ $\text{Oxygen typically has an oxidation state of } -2\text{.}$ $\text{Chlorine typically has an oxidation state of } -1\text{.}$ $x + 2 \times (-2) + 2 \times (-1) = 0$ $x - 4 - 2 = 0$ $x - 6 = 0$ $x = +6$ $\text{So, Cr is in the } +6 \text{ oxidation state.}$ $\text{Comparing the oxidation states:}$ $\text{MnO}_4^{-}: +7$ $\text{Cr}(\text{CN})_6^{3-}: +3$ $\text{NiF}_6^{2-}: +4$ $\text{CrO}_2\text{Cl}_2: +6$ $\text{The species with an atom in the } +6 \text{ oxidation state is } \text{CrO}_2\text{Cl}_2\text{.}$