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

The greater the size of the cation more is the ionic nature of hydrides.

And, Solubility $\propto$ Ionic nature

Size of cation order:

${\mathrm{Ba}}^{2+}>{\mathrm{Ca}}^{2+}>M{ \mathrm{g}}^{2+}>{\mathrm{Be}}^{2+}$

So, the ionic nature order is:

${\mathrm{BeF}}_2<{\mathrm{MgF}}_2<{\mathrm{CaF}}_2<{\mathrm{BaF}}_2$

Thus, the solubility of ${\mathrm{BaF}}_2$ is maximum in water.