The given temperature-time plot represents the change in temperature of a piece of naphthalene as it is being heated. It shows a curve that has a plateau or flat region. This plateau indicates that the temperature remains constant during this period, signifying a phase change.

Option 1 states that the plateau signifies the boiling point of naphthalene. However, the boiling point is the temperature at which a substance changes from a liquid to a gas. In the given plot, there is no indication of the naphthalene reaching its boiling point.

Option 2 suggests that the plateau represents the freezing point of naphthalene. However, the freezing point is the temperature at which a substance changes from a liquid to a solid. Again, there is no indication of the naphthalene reaching its freezing point in the given plot.

Option 3 correctly states that the plateau represents the melting point of naphthalene. The melting point is the temperature at which a substance changes from a solid to a liquid. The fact that the temperature remains constant during this plateau indicates that the naphthalene is undergoing a phase change from solid to liquid.

Option 4 suggests that the temperature at the plateau represents a chemical change upon heating. However, there is no information