Question 3
Sodium chloride melts at 801 °C while ammonia melts at −78 °C. Why do ionic compounds melt at such high temperatures?
The right answer is the one marked above.
Strong electrostatic attractions act throughout the whole lattice. Each ion is held by six neighbours, and melting means overcoming all of that, which takes a great deal of heat.
Mass is not what decides a melting point. There are no covalent bonds in an ionic compound to break, and a substance with no attractions between its particles could not be a solid at all.