ANS:D - all (a), (b) and (c).

The correct statement is: "The selectivity of solvent used in solvent extraction is unity at the plait point." The plait point refers to the point at which the equilibrium tie lines for a ternary liquid-liquid extraction system converge. At this point, the selectivity of the solvent becomes unity, meaning it has an equal affinity for both components being separated, resulting in no preferential extraction of either component. The other statements are incorrect:

• In case of liquid-liquid extraction, separation is indeed possible even if the selectivity of the solvent is unity. However, the separation efficiency may not be optimal.
• Typically, with an increase in temperature, the selectivity of the solvent used in solvent extraction increases, not decreases. This is because higher temperatures often enhance the solubility of one component in the solvent relative to the other, leading to improved selectivity.