Which one of the following alkali metals gives hydrated salts?

Smaller the size of an ion, the more highly is it hydrated. Among the given alkali metals, Li is the smallest in size. Also, it has the highest charge density and highest polarising power. Hence, it attracts water molecules more strongly than the other alkali metals. As a result, it forms hydrated salts such as

$\mathrm{LiCl} .2 \mathrm{H}_{2} \mathrm{O}$. The other alkali metals are larger than $\mathrm{Li}$ and have weaker charge densities. Hence, they usually do not form hydrated salts.