Mn ³⁺  and Co ³⁺  ions are the strongest oxidising agents in aqueous solution.

WHy?

Dear Student,

Outer electronic configuration of Mn is 3d⁵ 4s².

Outer electronic configuration of Mn³⁺ is 3d⁴ 4s⁰.

Now Mn³⁺ is a strong oxidizing agent. A good oxidizing agent reduces itself, i.e. gains electrons from others. Its tends to gain one more electron to acquire stable electronic configuration. If it gains one electron, its configuration will be 3d⁵, which is stable. This is the reason, it acts as a good reducing agent.

Outer electronic configuration of CO is 3d⁷ 4s².

Outer electronic configuration of CO³⁺ is 3d⁶ 4s⁰.

Now Co in CO³⁺ state is not stable in aqueous solution. But Co is stable in CO²⁺ state in water. This is the reason it likes to go from Co(III) to Co(II) state by gaining electrons or in other words, it gets reduced and acts as  good oxidizing agent.

Hope you got the answer!