ANS:D - The end points of a vapor-pressure vs. temperature curve are critical and triple points.

The correct statement is: "The end points of a vapor-pressure vs. temperature curve are critical and triple points." In the context of phase diagrams and vapor-pressure curves, the critical point is the highest temperature and pressure at which a distinct liquid and gas phase can coexist. The triple point is the unique combination of pressure and temperature at which three phases (solid, liquid, and gas) can coexist in equilibrium. Therefore, these two points mark the extremes of the vapor-pressure vs. temperature curve. Let's examine the other statements:

1. A substance existing above its critical temperature is called a saturated vapor: This statement is incorrect. Above its critical temperature, a substance can no longer exist as a liquid, and it forms a supercritical fluid, not a saturated vapor.
2. A mixture of vapor gas is called saturated if the equilibrium vapor pressure of the liquid is more than the partial pressure of the vapor at the same temperature: This statement is incorrect. In a saturated vapor, the equilibrium vapor pressure of the liquid is equal to the partial pressure of the vapor at the same temperature, not more than.
3. Heat added to or given up by a substance at constant temperature is called sensible heat: This statement is incorrect. Sensible heat refers to the heat exchanged by a substance resulting in a change in temperature, not necessarily at constant temperature. Heat added or removed at constant temperature is typically associated with latent heat, as it causes a change in phase rather than a change in temperature.