Final answer:
The boundaries on a temperature-composition phase diagram represent the vapor-liquid equilibrium, which is where a liquid and its vapor coexist in balance. The correct answer to the question is A) Vapor-liquid equilibrium.
Step-by-step explanation:
A temperature-composition diagram is a type of phase diagram that illustrates the equilibrium between different phases of a substance as a function of both temperature and composition, typically when different components are present. This type of phase diagram can show various lines that represent the conditions (temperature and pressure) at which different phases can coexist in equilibrium, such as solid-liquid, liquid-gas, and solid-gas transitions. In particular, the boundaries on this diagram show the state of equilibrium between the two phases.
For the specific boundaries mentioned in this question, the correct answer is A) Vapor-liquid equilibrium. This boundary represents the conditions under which a liquid and its vapor (the gas phase of the same substance) are in equilibrium. As the boundary is crossed, the substance may condense or vaporize, which are transitions between the liquid and gas phases.