The halide that is highly soluble in ${\mathrm{H}}_2\mathrm{O}$  among the following is

The relation between hydration energy and size of cation are related as given below 

Hydration energy ∝ $\frac{1}{\mathrm{size}}$

Size:- ${\mathrm{Be}}^{+2}<{\mathrm{Ca}}^{+2}<{\mathrm{Sr}}^{+2}<{\mathrm{Be}}^{+2}$

Hydration energy :-${\mathrm{Be}}^{+2}>{\mathrm{Ca}}^{+2}>{\mathrm{Sr}}^{+2}>{\mathrm{Ba}}^{+2}$

Solubility:- ${\mathrm{BeF}}_2>{\mathrm{CaCl}}_2<{\mathrm{SrCl}}_2<{\mathrm{BaCl}}_2$