A reducing agent donates electrons and is itself oxidised, so the strongest reducing agent is the metal with the most negative standard reduction potential. The standard reduction potentials are approximately Mg^2+/Mg = -2.37 V, Zn^2+/Zn = -0.76 V, Cu^2+/Cu = +0.34 V, and Ag^+/Ag = +0.80 V. The more negative the value, the greater the tendency of the metal to lose electrons and be oxidised, hence the stronger its reducing power. Magnesium has the most negative potential (-2.37 V), so it is the strongest reducing agent. Copper is incorrect because its positive potential makes it a poor electron donor. Silver, with the most positive potential, is the weakest reducing agent and the best oxidising cation. Zinc reduces more easily than copper or silver but less readily than magnesium. This ordering reflects the electrochemical series central to NCERT electrochemistry. Plausibility check: the activity series ranks Mg above Zn above Cu above Ag, fully consistent with the chosen answer.