Which one among the following metals is the weakest reducing agent?

The reducing strength of an alkali metal in aqueous solution depends on its standard reduction potential ((E^\circ)). A more negative reduction potential corresponds to a stronger reducing agent, while a less negative value corresponds to a weaker reducing agent.

The standard reduction potential is influenced by sublimation energy, ionization energy, and hydration energy. Although lithium has the highest ionization energy among the alkali metals, the exceptionally large hydration energy of the small (Li^+) ion more than compensates for it. As a result, lithium has the most negative standard reduction potential ((E^\circ = -3.04\ \text{V})) and is therefore the strongest reducing agent in aqueous solution.

Among the given options, potassium, sodium, and rubidium have less negative standard reduction potentials than lithium. Hence, lithium cannot be the weakest reducing agent.

Therefore, the answer key indicating (Li) as the weakest reducing agent is incorrect. In aqueous solution, (Li) is actually the strongest reducing agent due to its very high hydration enthalpy.