The correct answer is option 4: Atmospheric scattering.
Atmospheric scattering is the phenomenon that causes the sun to be visible a little before the actual sunrise. This is because when sunlight passes through the Earth`s atmosphere, it interacts with the molecules and particles in the air. These interactions cause the sunlight to scatter in different directions, and some of this scattered light reaches our eyes, allowing us to see the sun even when it is still below the horizon.
Option 1: Atmospheric reflection refers to the bouncing back of light from a surface. While reflection does occur in the atmosphere, it is not the main reason why the sun is visible before sunrise.
Option 2: Atmospheric dispersion refers to the process of separating light into its different colors. While dispersion does occur in the atmosphere, it is not the main reason why the sun is visible before sunrise.
Option 3: Atmospheric diffraction refers to the bending of light around obstacles. While diffraction does occur in the atmosphere, it is not the main reason why the sun is visible before sunrise.
Therefore, the correct explanation is that the sun is visible a little before the actual sunrise due to atmospheric scattering.