The correct answer is: D. Air temperature
As a parcel of air rises, it expands and cools adiabatically. This means that the temperature of the air parcel decreases at a rate of about 10 degrees Celsius per kilometer of ascent. This is because the pressure of the air decreases as it rises, and the air molecules have more space to move around. As they move around more, they collide with each other less often, and the temperature of the air decreases.
Saturation vapor pressure is the maximum amount of water vapor that can be held in the air at a given temperature and pressure. As the air parcel rises, the pressure decreases and the temperature decreases. This causes the saturation vapor pressure to decrease as well.
Relative humidity is the ratio of the actual amount of water vapor in the air to the saturation vapor pressure. As the air parcel rises, the relative humidity increases. This is because the saturation vapor pressure decreases as the air parcel rises, but the actual amount of water vapor in the air remains constant.
Mixing ratio is the ratio of the mass of water vapor to the mass of dry air. As the air parcel rises, the mixing ratio remains constant. This is because the mass of water vapor in the air parcel remains constant, and the mass of dry air in the air parcel decreases as the air parcel rises.
Therefore, the only option that will increase in a rising parcel of air is air temperature.