To determine the correct option, we should evaluate the oxidation states of the metals in the given compounds to see in which cases the metals are in their highest oxidation states.

Option A: MnO₂, FeCl₃

For MnO₂, manganese (Mn) has an oxidation state of +4. While this is a high oxidation state for Mn, it is not its highest, as it can go up to +7 (as in KMnO₄).

For FeCl₃, iron (Fe) has an oxidation state of +3, which is its highest stable oxidation state.

So, this option is not entirely correct.

Option B: [MnO₄]^-, CrO₂Cl₂

For [MnO₄]^-, Mn is in the +7 oxidation state, which is indeed its highest oxidation state.

In CrO₂Cl₂, chromium (Cr) is in the +6 oxidation state, which is its highest oxidation state.

Both Mn and Cr are in their highest oxidation states, making this option correct.

Option C: [Fe(CN)₆]^3-, [Co(CN)₆]^3-

For [Fe(CN)₆]^3-, Fe is in the +3 oxidation state, its highest stable oxidation state.

However, [Co(CN)₆]^3- seems to be incorrectly provided as it suggests an oxidation number that does not fit standard cobalt complexes or could be a typographical error because typically, cobalt in a cyanide complex like [Co(CN)₆]^3- would suggest an oxidation state of +3 for cobalt. Cobalt can exist in +2 and +3 oxidation states commonly, with +3 being very stable in this coordination compound. But it isn't necessarily the highest oxidation state Co can achieve (it could theoretically go up to +4, but such compounds are very rare and not stable).

Therefore, there seems to be a misunderstanding in the elements present or typo in this option for Co. Considering the usual cobalt complex and common oxidation states, though Co is in a high oxidation state (+3), the context seems mixed up, and noting the rarity and instability of Co(IV) compounds, the focus should rather be on the correctness of available options against the question's criteria.

Option D: [NiCl₄]^2-, [CoCl₄]^-

For [NiCl₄]^2-, Ni is in the +2 oxidation state, which is not its highest (it can go up to +3).

The [CoCl₄]^- mistakenly listed suggests cobalt in an oxidation state lower than its maximum, specifically +2, considering the anion's charge and typical coordination chemistry. Like Ni, Co in +2 is common but not its highest.

This option does not have metals in their highest oxidation states either.

Conclusion

Option B, with [MnO₄]^- and CrO₂Cl₂, correctly presents the metals (Mn and Cr) in their highest oxidation states, making it the correct choice.