KMnO4can be prepared fromK2MnO4as per the reaction3MnO42−+2H2O⇌2MnO4−+MnO2+4OH−The reaction can go to completion by removing OH− ions by adding

HCl and SO2 are reducing agents. So they can reduce  MnO4−. CO2is neither oxidising nor reducing agent, it will provide only acidic medium. It can shift the reaction in forward direction and the reaction can go to completion.