Which of the following compounds has the lowest melting point?

According to Fajan's rules, the covalent character of an ionic compound increases with the size of the anion, given that the cation remains the same. As we move down the halogen group, the size of the halide ion increases in the order $\text{F}^- < \text{Cl}^- < \text{Br}^- < \text{I}^-$. The larger the anion, the more easily it is polarised by the cation, leading to greater covalent character. Therefore, $\text{CaI}_2$ exhibits the maximum covalent character among the given options. Since covalent compounds generally have weaker intermolecular forces and lower melting points compared to the strong electrostatic forces in ionic compounds, $\text{CaI}_2$ has the lowest melting point.