Which of the following ionic solids has the lowest melting point?
  A.KCl
  B.NaCl
  C.LiF
  D.LiCl

Hint: The temperature at which a substance undergoes a change of phase from solid to liquid without an increase in temperature is called its melting point. It can also be defined as the temperature at which the solid and liquid phases of a substance exist at equilibrium.

Complete step by step answer:
-Solids that are formed by formation of an ionic bond are called ionic solids.
-Ionic substances are formed by formation of ionic bonds. An ionic bond is one which is formed by donation of an electron from one atom and acceptance of the same electron by the other atom.
-Melting point of an ionic compound is generally high because the electrostatic attraction between the cations and anions is strong. The energy required to overcome this force of attraction is high. It is so because this energy is used to make two oppositely charged ions move away from each other and change the phase from solid to liquid.
-Many factors affect the melting point of ionic compounds. They are –
Charge on ions – The charge on an ion is directly proportional to the electrostatic attraction. The strength of the ionic bond is directly proportional to the charge on an ion. Thus, the stronger the ionic bond, higher will be the melting point.
The size of ions – Smaller ions get packed closely as compared to the larger ions. The electrostatic attraction is greater in smaller ions and the ionic bond is stronger. Hence, the melting point of a small ion is higher. The radii of ions are inversely proportional to melting points.
-In this situation, the largest cation is Li. So, the least melting point will be of one of the two compounds, \[LiCl\] or $LiF$. Out of \[LiCl\] and $LiF$, the size of fluoride ions is smaller than the size of the chloride ion. So, the minimum melting point will be of \[LiCl\].

Hence option D is correct.

Note:
-Melting point depends on size, it decreases down the group. It depends on charge, it increases across the period.
-We need to consider the charge and size of both the anion and the cation.