ANS:C - both (a) and (b).

In the case of an unsaturated vapor-gas mixture, the humid volume increases with an increase in the absolute humidity at a given temperature. Here's why:

1. Absolute Humidity: Absolute humidity refers to the actual amount of water vapor present in the air. If the absolute humidity increases (more water vapor is added to the air), the air becomes more humid.
2. Humid Volume: Humid volume is a measure of the volume occupied by a unit mass of a mixture of dry air and water vapor. As the absolute humidity increases, more water vapor molecules are present in the mixture, causing the mixture to occupy a larger volume.
3. Total Pressure: The total pressure of the mixture affects the volume through the ideal gas law, but it doesn't directly influence the amount of water vapor present in the mixture.