ANS:D - none of these.

The cooling effect in a cooling tower can not be increased by lowering the barometric pressure. Cooling towers operate based on the principle of evaporative cooling, where warm water is brought into contact with air. As the water evaporates, it absorbs heat from the surroundings, cooling the water and the air. Let's consider the options:

1. Increasing the air velocity over the wet surfaces: This can increase the rate of evaporation, leading to enhanced cooling effects.
2. Reducing the humidity of entering air: Lower humidity means a larger difference in vapor pressure between the water in the tower and the surrounding air, which promotes more evaporation and greater cooling effects.
3. Lowering the barometric pressure: Lowering the barometric pressure would actually decrease the partial pressure of water vapor in the air, which could lead to increased evaporation rates and enhanced cooling effects, especially if the water is near saturation. However, lowering the barometric pressure is not something that can be practically controlled in a cooling tower operation, as it is determined by atmospheric conditions.