Answer:
The Horizontal line
Step-by-step explanation:
A heating curve is usually defined as a graphical representation of transition of phases. This shows how a substance changes it state from one type to another. It commonly shows the transition phases of water, namely the solid, liquid and the gaseous state. The diagonal or the inclined lines shows the changes in terms of temperature, and the horizontal lines shows the changing of phases.
When any solid is allowed to get heated at its melting point, then there occurs this horizontal line or the flat line, showing that the temperature is constant until and unless the solid is fully melted, and this happens due to the requirement of high amount of energy for the melting process to take place.