The solvent used in liquid-liquid extraction should have __________ less than one.

ANS:D - neither (a) nor (b).

If the solvent used in liquid-liquid extraction had both selectivity and distribution coefficient less than one, it would mean that the solvent has a lower affinity for both components in the mixture compared to the feed solution. This scenario would not be suitable for effective extraction because the solvent would not preferentially extract any component from the mixture. As a result, neither selectivity nor distribution coefficient less than one would be desirable for efficient separation in liquid-liquid extraction. Therefore, the statement "neither (a) nor (b)" implies that neither selectivity nor distribution coefficient less than one would be appropriate for the solvent used in liquid-liquid extraction.