Which of the following compounds is highly soluble in water?

$\text{CSOH}>\text{RbOH}>\text{KOH}>\text{NaOH}>\text{LiOH}$

The solubility of both oxides and hydroxides increases down the group. Going down the group both hydration enthalpy, lattice enthalpy decreases down the group but lattice enthalpy decreases down faster so, solubility of oxides, hydroxides decreases down the groups.