Which solid will have the weakest intermolecular forces?

The solid phosphorus pentachloride exists as:

The lattice enthalpy of an ionic compound is the enthalpy change when one mole of an ionic compound in its solid state dissociates into its ions in the gaseous state. It is impossible to determine it directly by experiment. Suggest and explain an indirect method to measure the lattice enthalpy of $NaCl_{(s)}$.

Which of the following exhibits the weakest intermolecular forces?

The ionic radii of $Rb^{+}$ and $I^{-}$ are $1.46 \ \mathring{A}$ and $2.16 \ \mathring{A}$,respectively. The most probable type of structure exhibited by it is:

Match the type of interaction in Column $I$ with the distance dependence of their interaction energy in Column $II$:

$I$. Ion-ion	$a$. $1/r$
$II$. Dipole-dipole	$b$. $1/r^{2}$
$III$. London dispersion	$c$. $1/r^{3}$
	$d$. $1/r^{6}$