Which of the following compounds has the lowest melting point?

According to Fajan's rules, as the size of the anion increases (${\text{F}}^{-}<{\text{Cl}}^{-}<{\text{Br}}^{-}<{\text{I}}^{-}$), its polarizability increases, leading to more covalent character in the ionic bond. Covalent compounds generally have lower melting points than ionic ones. Thus, ${\text{CaI}}_2$ has the highest covalent character and the lowest melting point.