Which of the following compounds has the lowest melting point?
A. $CaC{l_2}$
B. $CaB{r_2}$
C. $Ca{I_2}$
D. $Ca{F_2}$

Hint: The melting point of a substance is the temperature at which it changes from solid state to liquid state. The melting point of a compound is directly proportional to its lattice energy. The lattice energy of a solid compound is the amount of energy released when the respective ions are combined to form it. Lattice energy is inversely proportional to the size of the compound.

Complete step by step answer:
We are given to find the compound with the lowest melting point from among the given options.
The compounds given are Calcium chloride ($CaC{l_2}$), calcium bromide ($CaB{r_2}$), calcium iodide ($Ca{I_2}$) and calcium fluoride ($Ca{F_2}$).
Fluorine, Chlorine, bromine and Iodine belong to the 17th group elements and are called halogens.
The atomic number of fluorine is 9, chlorine is 17, bromine is 35 and iodine is 53.
The size of iodine is greater than bromine, the size of bromine is greater than chlorine, and the size of chlorine is greater than fluorine.
This means the lattice energy of calcium iodide ($Ca{I_2}$) is the least compared to Calcium chloride ($CaC{l_2}$), calcium bromide ($CaB{r_2}$) and calcium fluoride ($Ca{F_2}$).
This means that calcium iodide ($Ca{I_2}$) has the lowest melting point.

Therefore, the correct option is Option C, calcium iodide ($Ca{I_2}$)

Note:
Do not confuse lattice energy with hydration energy. Lattice energy is the amount of energy released when ions combine to make a compound whereas hydration energy is the amount of energy released when one mole of ions undergoes hydration (treating with water). Both energies are directly proportional to the charge present on ions and inversely proportional to the size of ions.