The correct answer is: D. Water and diethyl aniline.
A lower critical solution temperature (LCST) is the temperature below which two liquids that are immiscible at higher temperatures become miscible. An upper critical solution temperature (UCST) is the temperature above which two liquids that are miscible at lower temperatures become immiscible.
Water and diethyl aniline are a partially miscible liquid pair that has both an LCST and an UCST. This means that at temperatures below the LCST, water and diethyl aniline are immiscible, but at temperatures above the LCST, they are miscible. The UCST for water and diethyl aniline is approximately 293 K.
The reason why water and diethyl aniline have both an LCST and an UCST is because of the difference in their polarity. Water is a polar molecule, while diethyl aniline is a nonpolar molecule. When water and diethyl aniline are mixed at low temperatures, the water molecules form hydrogen bonds with each other, while the diethyl aniline molecules do not interact with each other. This results in the two liquids being immiscible.
However, as the temperature increases, the water molecules begin to move faster and have more energy. This allows them to break the hydrogen bonds that they have formed with each other. As a result, the water molecules become more and more able to interact with the diethyl aniline molecules. At the UCST, the water molecules and the diethyl aniline molecules are able to interact with each other equally, and the two liquids become miscible.
Above the UCST, the water molecules and the diethyl aniline molecules continue to move faster and have more energy. This allows them to interact with each other even more strongly. As a result, the two liquids become even more miscible.
The LCST and UCST for water and diethyl aniline can be affected by the addition of other substances. For example, if a salt is added to water and diethyl aniline, the LCST will be lowered and the UCST will be raised. This is because the salt ions will interfere with the hydrogen bonds that the water molecules form with each other. As a result, the water molecules will be less able to interact with each other, and the two liquids will become more miscible at lower temperatures.