Among the given metals, the one with the lowest (most negative) standard reduction potential ($E^{\circ }$) is the strongest reducing agent and can reduce $F{e}^{3+}$ to $F{e}^{2+}$.

• Target couple: $F{e}^{3+}+e^{-}\to F{e}^{2+}$ ($E^{\circ }=+0.77\text{ V}$)
• Standard reduction potentials of the given metals:
  • $Z{n}^{2+}/Zn$: $E^{\circ }=-0.76\text{ V}$
  • $C{u}^{2+}/Cu$: $E^{\circ }=+0.34\text{ V}$
  • $A{g}^{+}/Ag$: $E^{\circ }=+0.80\text{ V}$
  • $A{u}^{3+}/Au$: $E^{\circ }=+1.50\text{ V}$
• Selection: $Zn$ has the lowest $E^{\circ }$ ($-0.76\text{ V}$), making it the strongest reducing agent among the options. Since $E_{Zn}^{\circ }<E_{F{e}^{3+}/F{e}^{2+}}^{\circ }$, $Zn$ will spontaneously reduce $F{e}^{3+}$ to $F{e}^{2+}$.