Manganese is present in the highest oxidation state in:

Correct answer: (b) KMnO₄

Explanation

Taking oxygen as −2 and potassium as +1, the oxidation state (x) of manganese in each is:

• MnO₂: x + 2(−2) = 0, so x = +4;
• KMnO₄: +1 + x + 4(−2) = 0, so x = +7;
• Mn₂(CO)₁₀: CO is a neutral ligand, so x = 0;
• MnO₄²⁻ (manganate): x + 4(−2) = −2, so x = +6.

So manganese is in its highest oxidation state, +7, in potassium permanganate. This is also the highest state manganese shows, which is why KMnO₄ is a strong oxidising agent.