The most soluble hydroxide among the following is

Solubility : Be(OH)2 < Mg(OH)2 < Ca(OH)2 < Sr(OH)2 < Ba(OH)2As lattice energy decreases solubility increasesIf anion size is small    then with small change in cation size , then there is a considerable change in lattice energy. And hence in this case the solubility depends upon the lattice energyIn a series of compounds with increases in the size of oppositely charges ion lattice energy decreases there by solubility increases