Final answer:
The salt dissolves when heating the water because the increased temperature elevates the solubility of the salt, causing more rapid and forceful molecular collisions, which accelerates the dissolution rate. The correct option is D. Increasing the temperature increases the solubility of salt.
Step-by-step explanation:
When Tim adds salt to the water and heats it, the salt dissolves because increasing the temperature increases the solubility of the salt. Heating up a solvent like water gives the water molecules more kinetic energy, causing them to move more rapidly and collide more frequently and with greater force with the salt molecules.
This action increases the rate at which the solute dissolves. Furthermore, as the temperature increases, it is typical for the solubility of solid substances to increase, thus allowing more salt to dissolve in the boiling water.