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

The reactivity of alkali metals increases down the group. This is because as we move down the group, the atomic size increases, and the outermost electron is more loosely held, making it easier to donate and hence, increasing its reactivity.

In the given options, K (Potassium), Rb (Rubidium), Na (Sodium), and Li (Lithium), Li is at the top of the group and hence, is the least reactive. Therefore, Lithium (Li) is the weakest reducing agent among the given metals.