This point is the upper limit of the vapor-pressure line.

• A. Triple Point
• B. Critical Point
• C. Boiling Point
• D. Saturation Point

Answer: (B)

The point where the liquid–vapor boundary ends is the critical point. On a phase diagram, the vapor-pressure line shows the equilibrium between liquid and its vapor. As temperature rises, this boundary vanishes at the critical point, where liquid and vapor become indistinguishable and a supercritical fluid forms. That’s why it marks the upper limit of the vapor-pressure line.

The triple point is where solid, liquid, and gas coexist, a separate feature lower on the diagram. The boiling point is the temperature at which a liquid’s vapor pressure equals the external pressure, not the endpoint of the liquid–vapor boundary. Saturation point isn’t the standard term for this endpoint.