ANS:A - loosing sensible heat.

In a cooling tower, water becomes cool primarily through the process of evaporative cooling, which involves the vaporization of a small portion of water due to heat loss to the air. When warm water is introduced into the cooling tower, some of it is sprayed or allowed to flow over surfaces within the tower. As air flows through the tower, it comes into contact with the water. Heat from the water is transferred to the air through evaporation, causing a small portion of the water to evaporate and turn into vapor. This phase change from liquid to vapor absorbs heat energy from the remaining water, thereby cooling it down. Therefore, the correct option is: vaporization due to heat loss to air.