Answer:
C) Grind the rock salt into smaller pieces.
Step-by-step explanation:
To dissolve rock salt we need certain amount of heat, which is provided by the warm water. Now, this warm water won't dissolve easily the rock salt, because that salt in that state as rock, it's harder, that is, the heat won't penetrate easily in to the material to dissolve it.
Now, if we grind the rock salt into smaller pieces, we'll accelerate the process, because each pieces requires less heat to be dissolved. Basically, when we do smaller pieces, we are giving the energy needed to grind that rock, and such energy won't be needed from the warm water, so the actual energy from the warm water will be focused on dissolving each small piece.
Therefore, the right answer is C.