Why is the freezing point of a solution, LOWER than that of the pure solvent?
Because freezing point depression operates.....
The addition of a salt, any salt, to a solvent decreases the freezing point of that solvent by an amount proportional to the concentration of that salt in solution. This is a well-known colligative property. And thus addition of salt to water decreases the melting point below
You should look up colligative properties in your text: these include (i) freezing point depression; (ii) boiling point elevation; and (iii) osmotic potential.