The standard reduction potential at 25^oC for the following half-reaction are given below:

Zn⁺²(aq.) + 2e^– → Zn(s);       -0.76 volt

Cr⁺³ (aq.) + 3e^–→ Cr(s);        -0.740 volt

Cu⁺²(aq.) + 2e^– → Cu(s);       0.34 volt

Fe⁺³ + e– → Fe⁺²  ;                 0.77 volt

Which is the strongest reducing agent

Out of the following given half-reaction, the reduction of Zn⁺² has the lowest reduction potential(-0.762). We know that

Oxidation  potential = -(reduction potential)

So in terms of standard oxidation potential zinc will have the highest oxidation potential i.e, 0.762 volts. Therefore, zinc is the strongest reducing agent.