The reducing power of a metal depends on various factors. Suggest the factor which makes lithium, the strongest reducing agent in aqueous solution.

Reducing property of a metal depends upon various factors
1) sublimation enthalpy [IE]
    

2) Ionisation energy [IE]
    

3) Hydration enthalpy: [HE]
   

Size of M⁺ ions : Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺
H.E Order :${\mathrm{Li}}^{+}>{\mathrm{Na}}^{+}>{\mathrm{K}}^{+}>{\mathrm{Rb}}^{+}>{\mathrm{Cs}}^{+}$

As the size of M⁺ ion ↑, Nuclear charge ↓^es and hence attractive forces on the water molecule ↓^es, and hence degree of hydration ↓^es
Therefore hydration energy ↓^es
i.e Li⁺ ion has highest hydration energy
sublimation energy (S.E) & Ionisation energies [IE] are absorbed energies,
where as hydration energy [H.E] is released energy
In the case of Li
 

As high H.E of "Li" it can easily gives the electrons in large number, therefore it can acts as strongest reducing agent in aqueous solutions among [alkali] metals in the periodic table.