Under what condition will clouds, fog, or dew form?

Clouds, fog, or dew form primarily through the process of condensation of water vapor in the atmosphere. When air becomes saturated with moisture, typically due to cooling, the excess water vapor condenses into tiny liquid water droplets, leading to the formation of clouds or dew. This process requires the presence of water vapor in the air and a temperature drop that allows the air to reach its dew point, the temperature at which air becomes saturated.

In this context, the other conditions listed do not directly lead to the formation of clouds, fog, or dew in the same way. Heating the air may cause it to rise and can actually prevent condensation by reducing humidity. Increasing air pressure can lead to clearer conditions by encouraging air to sink and dry out rather than supporting saturation. While compressing air can increase temperature and pressure, condensation occurs when air is cooled, leading to the opposite effects needed for fog, dew, or cloud formation. Therefore, the key process involved in the formation of these phenomena is the condensation of water vapor, making it the correct answer.