The tendency of forming chloride hydrates of alkaline earth metals is in the order of

Halides of alkaline earth metals form hydrate, so they are hygroscopic in nature. Due to this nature, they can be used as water absorbing compound. $\mathrm{Mg}$ is smaller than $\mathrm{Ca}$. Hence, $\mathrm{Mg}^{2+}$ ion polarises water molecule easily than the calcium water molecules get attached to $\mathrm{Mg}^{2+}$ as the water of crystallisation. Similarly $\mathrm{Sr}$ is smaller than $\mathrm{Ba}$ and easily polarises by water molecule. $$ \begin{aligned} & \text { i.e. } \mathrm{MgCl}_2 \cdot 6 \mathrm{H}_2 \mathrm{O}>\mathrm{CaCl}_2 \cdot 6 \mathrm{H}_2 \mathrm{O} \text {, } \\ & \mathrm{SrCl}_2 \cdot 2 \mathrm{H}_2 \mathrm{O}>\mathrm{BaCl}_2 \cdot 2 \mathrm{H}_2 \mathrm{O} \end{aligned} $$ Hence, correct order is $\mathrm{Mg}>\mathrm{Ca}$ and $\mathrm{Sr}>\mathrm{Ba}$.