Question

if a solid piece of naphthalene is heated and remains at 80c until it is compleatly melted you know that 80 c is

Answer 1

its just like
Water freezes at 0°C.
Ice melts at 0°C.
the freezing point and melting point are both two sides of the same coin just depending on which direction you are going.
so 80°C is both the melting point and the freezing point.

Answer 2

both the freezing and melting points of naphthalene