Step 1: Understanding Reducing Power and \( E^\circ \) Values
- The standard reduction potential (\( E^\circ \)) of alkali metals determines their ability to lose electrons and act as reducing agents.
- A more negative \( E^\circ \) value indicates a stronger reducing agent.
- The standard reduction potentials (\( E^\circ \)) of alkali metals are:
\[
Li = -3.04 V, \quad Na = -2.71 V, \quad K = -2.93 V, \quad Cs = -2.92 V
\]
Step 2: Identifying the Weakest Reducing Agent
- The reducing power of alkali metals increases down the group due to decreasing ionization energy.
- Lithium (\( Li \)) has the most negative \( E^\circ \) value, making it the strongest reducing agent.
- Sodium (\( Na \)) has the least negative \( E^\circ \) value among the given options, meaning it is the weakest reducing agent.
Conclusion: Sodium (Na) is the weakest reducing agent as per its \( E^\circ \) value.
Thus, the correct answer is Option (4): \( Na \).