Concept:The fixed temperature at which a liquid changes into gas is the boiling point.Explanation:At the boiling point, the vapour pressure of the liquid equals the surrounding atmospheric pressure.At this constant temperature, the liquid changes into the gaseous state throughout its entire volume.Unlike evaporation, which occurs only at the surface and at any temperature, boiling happens at one specific temperature for a given pressure.Therefore, the constant temperature at which a liquid changes into the gaseous state is the boiling point.Answer:B. boiling point