Which hydroxide has the lowest solubility in water at 298 K among the following?

Solubility of the Group 2 hydroxides increases down the group, because lattice enthalpy falls faster than hydration enthalpy as the cation grows. $\mathrm{Be}(\mathrm{OH}){}_2$ is at the top and is practically insoluble, while $\mathrm{Ba}(\mathrm{OH}){}_2$ is clearly soluble.