ANS:C - is a function of the liquid phase composition.

The correct statement about the activity coefficient of a solution, which accounts for the departure of the liquid phase from ideal solution behavior, is: "The activity coefficient measures the departure of the liquid phase from ideal solution behavior." This means that the activity coefficient is a measure of how much the behavior of a real solution deviates from that of an ideal solution. Ideal solutions follow Raoult's law, where the vapor pressure of each component is directly proportional to its mole fraction in the solution. However, real solutions often deviate from this behavior due to interactions between molecules. Now, let's analyze the given options:

1. "Measures the elevation in boiling point." - This statement is incorrect. The activity coefficient is not directly related to the elevation in boiling point. The boiling point elevation is a colligative property, which depends on the concentration of solute particles in the solution.
2. "Is not dependent on the temperature." - This statement is incorrect. The activity coefficient is temperature-dependent. As temperature changes, the interactions between solute and solvent molecules change, affecting the activity coefficient.
3. "Is a function of the liquid phase composition." - This statement is correct. The activity coefficient depends on the composition of the liquid phase, including the identities and concentrations of the solute and solvent.
4. "Measures the depression in freezing point." - This statement is incorrect. The depression in freezing point is another colligative property, which depends on the concentration of solute particles in the solution.