Step 1: Recall the meaning of reducing agent.
A reducing agent is a substance that loses electrons and causes reduction of another species. The stronger the tendency of a metal to lose electrons, the stronger is its reducing character. On the other hand, a metal that does not lose electrons easily behaves as a weak reducing agent.
Step 2: Use standard reduction potentials.
The tendency of a metal to act as a reducing agent can be understood from its standard reduction potential \(E^\circ\). A more negative reduction potential means the metal more readily undergoes oxidation, so it is a stronger reducing agent. A more positive reduction potential means the metal resists oxidation and is therefore a weaker reducing agent.
From the given values:
\[
E^\circ_{(\mathrm{Li^+/Li})} = -3.05\ \mathrm{V}
\]
\[
E^\circ_{(\mathrm{Ag^+/Ag})} = 0.80\ \mathrm{V}
\]
\[
E^\circ_{(\mathrm{Au^{3+}/Au})} = 1.40\ \mathrm{V}
\]
\[
E^\circ_{(\mathrm{Mg^{2+}/Mg})} = -2.36\ \mathrm{V}
\]
Step 3: Compare the metals.
Li: Very negative \(E^\circ\), so it is a very strong reducing agent.
Mg: Also has negative \(E^\circ\), so it is a strong reducing agent.
Ag: Positive \(E^\circ\), so it is weaker than Li and Mg as a reducing agent.
Au: Has the highest positive \(E^\circ\) value, so it has the least tendency to lose electrons and is therefore the weakest reducing agent.
Step 4: Conclusion.
Since gold has the highest positive standard reduction potential among the given metals, it behaves as the weakest reducing agent.
Final Answer:Au.