ANS:D - all (a), (b) & (c).

The enthalpy of a vapor gas mixture can indeed be increased by increasing the temperature at constant humidity, by increasing the humidity at constant temperature, or by simultaneously increasing both the temperature and humidity. Here's how each scenario affects the enthalpy:

1. Increasing temperature at constant humidity: When temperature increases, the kinetic energy of the gas molecules increases, leading to higher enthalpy.
2. Increasing humidity at constant temperature: Adding more water vapor to the mixture increases the number of water molecules present, which can increase the enthalpy due to the latent heat of vaporization.
3. Increasing both temperature and humidity: This combination amplifies the effect, as both factors contribute to raising the enthalpy of the vapor gas mixture.