ANS:D - on heating, a vapor phase will appear ; while on cooling, a solid phase will appear.

If a binary system has no critical solution temperature (CST), it implies that the system comprises of miscible liquids. Critical solution temperature (CST) is a characteristic of systems where the solubilities of components change with temperature. When the solubilities of components increase with temperature, the CST is the temperature above which they dissolve completely, and when the solubilities decrease with temperature, the CST is the temperature below which they dissolve completely. In a system with no CST, the solubilities do not exhibit such a behavior, indicating that the components are completely miscible over the entire temperature range. Therefore, the system comprises of miscible liquids.