ANSWER
At 90 degrees Celcius the substance will melt completely
Step-by-step explanation
Melting point is the process in which solid is completely converted to the liquid phase.
In the given graph, the point A the solid particles are closely packed due to strong intermolecular forces binding the particles together. When temperature is applied to the system, the particles of the solid substance vibrate about a point and the particles are said to move independently. Further increase in the temperature will break the bond holding the particles together and turn to liquid at 90 degrees Celcius
Therefore, at 90 degrees Celcius the substance will melt completely and turn to liquid.