Why does molten sodium chloride conduct electricity?

The lattice energy of the lithium halides is in the following order:

Consider the Born-Haber cycle for the formation of an ionic compound given below and identify the compound $Z$ formed.
$M_{(s)}$ $\xrightarrow{\Delta H_1} M_{(g)}$ $\xrightarrow{\Delta H_2} M^{+}_{(g)}$
$\frac{1}{2} X_{2(g)}$ $\xrightarrow{\Delta H_3} X_{(g)}$ $\xrightarrow{\Delta H_4} X^{-}_{(g)}$
$M^{+}_{(g)} + X^{-}_{(g)} \xrightarrow{\Delta H_5} Z$

Which of the following compounds has an electrovalent linkage?

The principal reason that the melting point of $NaF$ is much higher than that of $RbBr$ is that

Which of the following compounds has the lowest melting point?