ANS:A - wet bulb

The minimum temperature to which water can be theoretically cooled down in a cooling tower by evaporative cooling is equal to the wet bulb temperature of the air. The wet bulb temperature represents the lowest temperature that can be achieved through evaporative cooling under a given set of atmospheric conditions. It accounts for the cooling effect produced by the evaporation of water into the air and is typically lower than the dry bulb temperature (the standard temperature measured with a regular thermometer). In a cooling tower, water is sprayed or dripped over a surface, and air is blown or drawn through the tower. As the air passes through the tower, it evaporates some of the water, which cools the remaining water. The temperature to which the water can cool down is limited by the wet bulb temperature of the incoming air. Once the water reaches the wet bulb temperature, further cooling through evaporative cooling becomes impossible. Therefore, the minimum temperature to which water can be cooled in a cooling tower is equal to the wet bulb temperature of the air.