Among $LiCl,RbCl,BeC{l}_2$ and $MgC{l}_2$, the compounds with greatest and least ionic character respectively are:

According to fazan's rule, ionic character directly depends on the size of cation and charge on the cation and inversely depends on the size of anion. Since here the anion ($C{l}^{-}$) is same, the ionic character depends only on the size and charge of the cation. Among the given chlorides, $Rb$ has the largest size and $Be$ has the smallest size. Hence, $B{e}^{+2}$ inspite of having greater charge (+2) will be least ionic because of very small size.

$\therefore$ $RbCl$ is most ionic and $BeC{l}_2$ is least ionic.