Answer is: Cr⁶⁺.
₂₄Cr 1s²2s²2p⁶3s²3p⁶3d⁵4s¹.
₂₄Cr⁶⁺ 1s²2s²2p⁶3s²3p⁶. 8 valence electrons.
₂₈Ni 1s²2s²2p⁶3s²3p⁶3d⁸4s².
₂₈Ni⁶⁺ 1s²2s²2p⁶3s²3p⁶3d⁴.
₂₆Fe 1s²2s²2p⁶3s²3p⁶3d⁶4s²
₂₆Fe⁶⁺1s²2s²2p⁶3s²3p⁶3d².
₂₇Co 1s²2s²2p⁶3s²3p⁶3d⁷4s²
₂₇Co⁶⁺ 1s²2s²2p⁶3s²3p⁶3d³.
₂₅Mn⁶⁺ 1s²2s²2p⁶3s²3p⁶3d¹.