ANS:A - constant boiling mixture.

Azeotropic distillation is employed to separate constant boiling mixtures. Azeotropic distillation is a specialized form of distillation used to break azeotropes, which are mixtures of liquids that boil at a constant temperature, resulting in a vapor phase composition that is the same as the liquid phase composition. These constant boiling mixtures pose challenges in traditional distillation processes because they cannot be easily separated by conventional means. In azeotropic distillation, additional components (such as entrainers or solvent) are added to the mixture to modify the vapor-liquid equilibrium, allowing for the separation of the components. By altering the composition of the vapor phase, it becomes possible to achieve separation even for constant boiling mixtures. Therefore, azeotropic distillation is employed specifically for separating constant boiling mixtures.