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

According to the electrochemical series

${K}^{+} / K \to - 2 . 93V$

${Rb}^{+} / Rb \to - 2 . 98V$

${Na}^{+} / Na \to - 2 . 71V$

${Li}^{+} / Li \to - 3 . 05V$

Among the given alkali metals, sodium metal is the weakest reducing agent as its standard reduction potential is least negative which is ${E}_{{Na}^{+} / Na}^{°} = - 2 . 71$

So, the correct option is C.