Question

temperature remain constant during melting of a solid -how will you defend this statement

Answer 1

temperature remain constant during the melting of a solid because the melting point of the solid has been already reached and now the further heat supplied is absorbred by the substance and store it as potential energy to overcome the force of molecules of the substance and become gas. So the temperature doesn't change because it is absorbed as latent heat of fusion.
Hope this can help you^^