Solubility is defined as the capability of the solute to dissolve in the solvent. Solubility depends upon the temperature. In the given graph, solubility of many salts is increases with increase in temperature but solubility of sodium chloride is relatively constant i.e. slightly effect of temperature.
In aqueous solution, sodium chloride completely exist as and ions. Thus, there is no external force required for its dissociation.
Hence, sodium chloride shows the least increase in solubility with increase in temperature.
