Concept:A horizontal plateau on a cooling curve shows a change of state at a constant temperature.Explanation:The molten substance cools until its temperature reaches 60∘C.At this temperature, the cooling curve becomes flat, meaning heat is being removed without a fall in temperature.This happens while the liquid changes into a solid.For a pure substance, the freezing point equals the melting point.Therefore, 60∘C represents the melting point of the substance.Answer:D. melting point
