What must happen for dew point to be reached in fog formation?

In fog formation, reaching the dew point is crucial because it is the temperature at which air becomes saturated with moisture, leading to condensation. This process requires that the air contains enough moisture. When moisture is added to the air, it increases the humidity level, causing the air to reach its saturation point more quickly. Once the dew point is achieved, the air can no longer hold all the moisture in vapor form, leading to the formation of tiny water droplets that we observe as fog.

While temperature decline can also effect dew point reach because cooler air holds less moisture, it is the addition of moisture that directly drives the saturation process necessary for fog formation. Hence, focusing on the necessity of moisture addition illuminates its primary role in the establishment of conditions for fog, corroborating why the answer is correct.