__________ is defined as the temperature to which the air would have to cool (at constant pressure and constant water vapour content) in order to reach saturation.

Dew point temperature 

The correct answer is option 1. Dew point temperature.

The temperature to which the air would have to cool (at constant pressure and constant water vapor content) in order to reach saturation is called Dew point temperature.

Dew point temperature is a measure of atmospheric moisture. It indicates the temperature at which air becomes saturated with water vapor, leading to condensation and the formation of dew, fog, or clouds. Dew point temperature is a critical parameter in meteorology and is used to assess the likelihood of precipitation, fog formation, and other weather-related phenomena. It is often used in conjunction with air temperature and relative humidity to provide a comprehensive picture of atmospheric conditions.