What type of bonds are present in molecular crystals?

If the value of ionic radius ratio $\left( \frac{r_c}{r_a} \right)$ is $0.52$ in an ionic compound,the geometrical arrangement of ions in the crystal is:

Among the following,the weakest one is:

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 $NaCl_{(s)}$.

Carbon dioxide is a gas,while $SiO_2$ is a solid because

The enhanced force of cohesion in metals is due to