The lightest element of group 7 (considering transition metals) is manganese (Mn).
An oxoanion with Mn in +6 oxidation state is manganate, MnO₄²⁻.
In manganate, Mn exhibits +6 oxidation state (where oxygen is -2, so Mn + 4(-2) = -2, giving Mn = +6).
The potassium salt K₂MnO₄ (potassium manganate) is green in color.
This is distinct from permanganate KMnO₄ (where Mn is +7) which is purple.
The characteristic green color of manganate solutions and salts makes this the answer.