ANS:D - Selectivity

For the choice of the separating agent in extractive distillation, the most important factor to be considered is selectivity. Selectivity refers to the ability of the separating agent to preferentially interact with one component of the mixture over others. In extractive distillation, the separating agent (also known as the solvent or entrainer) is added to the mixture to enhance the separation of the components by forming non-ideal liquid-liquid equilibria. The separating agent should have a high selectivity for the desired component(s) to effectively facilitate the separation process. While factors such as cost, availability, and toxicity are also important considerations in the selection of the separating agent, selectivity is paramount because it directly impacts the efficiency and effectiveness of the extractive distillation process. A highly selective separating agent will enable better separation of the components, leading to higher purity and yield of the desired product(s).