Which of the following impacts visibility when in fog?

Visibility in fog is primarily impacted by the accumulation of tiny water droplets suspended in the air, which scatter light and make it difficult to see clearly. When fog forms, it typically occurs near the ground, leading to significantly reduced visibility at low altitudes where pilots operate. Therefore, viewing this as a restriction on visibility aligns directly with option D, which highlights that visibility is compromised primarily in the lower altitudes where fog is concentrated.

The other options touch upon related atmospheric conditions but do not address the specific impact of fog on visibility as directly. For example, increased humidity and temperature/dew point spread can be contributing factors to the formation of fog, yet they don't inherently indicate a direct effect on visibility like fog itself does. Similarly, the notion of cool air rising rapidly describes a dynamic in atmospheric behavior that does not specifically pertain to how visibility is affected by fog.