Which of the following is least soluble in water?

 If abigger anion is attached to the alkali and alkaline earth metal groups , solubility decrease down the group . because solubilty depends on the the lattice energy  and hydration energy and in this case lattice energy > hydration energy . U _{lattice energy}  = ( Z⁺ Z^-) / (r ⁺ + r^-) and  U _{hydration energy}  = (Z⁺/r⁺) +( Z^-/r^- ) . Lattice energy increases down the group at faster rate than that of hydration energy . Hence, this option is correct.