ANS:D - heat of mixing.

The term for the heat of solution in a system where both the solute and solvent are liquids is called heat of mixing. Heat of mixing refers to the heat absorbed or released when two or more substances are mixed together to form a solution. This term encompasses the heat changes associated with the dissolution of solute molecules in the solvent, as well as any heat changes due to interactions between the solute and solvent molecules. The other options mentioned are specific to certain types of solutions:

1. Heat of solvation: This term specifically refers to the heat absorbed or released when solute molecules are dissolved in a solvent to form a solution. It is commonly used when the solute is a solid and the solvent is a liquid.
2. Heat of hydration: This term specifically refers to the heat absorbed or released when ions in a solute undergo hydration (surrounded by water molecules) upon dissolving in a solvent, usually water.
3. Standard integral heat of solution: This term is less commonly used and may refer to a specific standardized method or condition for measuring the heat of solution.