ANS:A - water has a high latent heat of evaporation.

The adiabatic saturation temperature and the wet bulb temperature are nearly equal for the air-water system under ambient conditions primarily because water has a high latent heat of evaporation. Here's why:

1. High Latent Heat of Evaporation: The wet bulb temperature is essentially the lowest temperature to which air can be cooled by the evaporation of water into it under adiabatic conditions. As water evaporates from the wet bulb, it absorbs heat from the surrounding air, causing the air temperature to decrease to the wet bulb temperature. Since water has a high latent heat of evaporation, it can absorb a significant amount of heat energy from the air during evaporation, cooling the air to a temperature close to the adiabatic saturation temperature.
2. Lewis Number: While the Lewis number does play a role in determining the ratio of heat and mass transfer rates in a system, it is not directly responsible for the equality between the adiabatic saturation temperature and the wet bulb temperature.
3. Equal under All Circumstances: The adiabatic saturation temperature and the wet bulb temperature are not always equal under all circumstances. However, under typical ambient conditions, where the air is not saturated with water vapor and there are no significant external influences, they are close to each other due to the reasons mentioned above.
4. Solubility of Components of Air in Water: While the solubility of components of air in water affects the vapor pressure of water and thus the equilibrium humidity level, it does not directly determine the equality between the adiabatic saturation temperature and the wet bulb temperature.

In summary, the high latent heat of evaporation of water is the primary reason why the adiabatic saturation temperature and the wet bulb temperature are nearly equal for the air-water system under ambient conditions. This allows water to effectively cool the air to a temperature close to the adiabatic saturation temperature during evaporation, resulting in the observed equality.