What type of bonds are present in molecular crystals?

Which of the following has the highest melting point?

$I_2$ molecules are held in the solid lattice by $...................$

Lattice enthalpy for $NaCl$ is $+788 \ kJ \ mol^{-1}$ and $\Delta H_{\text{hyd}}^{\circ} = -784 \ kJ \ mol^{-1}$. The enthalpy of solution of $NaCl$ is:

Suggest the most important type of intermolecular attractive interaction in the following pairs.
$(i)$ $n$-hexane and $n$-octane
$(ii)$ $I_{2}$ and $CCl_{4}$
$(iii)$ $NaClO_{4}$ and water
$(iv)$ methanol and acetone
$(v)$ acetonitrile $(CH_{3}CN)$ and acetone $(C_{3}H_{6}O)$

Alumina $(Al_2O_3)$ is: