The reduction potential for MnO4− to Mn2+ is +1.51V It can oxidise only those metals, which have reduction potential lower than +1.51V. For Zn2+∕Zn and Ag+∕Ag, reduction potential given is −0.762V and +0.80V respectively. These values are lower than reduction potential of MnO4−∕Mn2+ Hence, Zn and Ag will be oxidised. Since, for Au+∕Au, reduction potential is higher, hence it will not be oxidised by Mn.