The first one is 2
The second is 1
The third is 6
And the fourth is 3
The molecular formula shows the exact number of molecules. Therefor, the empirical formula is the simplest formula of the molecular formula
The composition would be more "diluted" in a sense.
It is true that substances that have a high melting point and conduct electricity in the liquid phase are ionic substances, and if one of your options is NaCl, then that is the correct answer.