$\mathrm{Na}^{+} < \mathrm{Zn}^{2+} < \mathrm{Cu}^{2+} < \mathrm{Ag}^{+}$ During electrolysis of aqueous solution, if a cation has higher electrode potential then water, it preferentially discharge at the cathode. The elements in the lower position in electro-chemical series gets discharged more easily, so $\mathrm{Na}^{+}$ discharged least preferentially and $\mathrm{Ag}^{+}$ more preferentially.