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

According to the electrochemical series

${\mathrm{K}}^{+} / \mathrm{K} \to -2.93\mathrm{V}$

${\mathrm{Rb}}^{+} / \mathrm{Rb} \to -2.98\mathrm{V}$

${\mathrm{Na}}^{+} / \mathrm{Na} \to -2.71\mathrm{V}$

${\mathrm{Li}}^{+} / \mathrm{Li} \to -3.05\mathrm{V}$

Among the given alkali metals, sodium metal is the weakest reducing agent as its standard reduction potential is least negative which is ${\mathrm{E}}_{{\mathrm{Na}}^{+}/\mathrm{Na}}^{°} = -2.71$

So, the correct option is C.