Hi ^-^
Your answer is: D. Melting Point
Explanation: Since we know that we are looking for a solid to a liquid, we know that there would be a melting point, Boiling point cannot be an answer because that would be a liquid to gas, solubility and hardness, just don't really have anything to do with the question.
Hope this helps and please mark as brainliest it would help a lot!
