Final answer:
The flattened appearance of the sun just before it sets is due to atmospheric refraction and scattering. The observer's position on Earth affects the visual perception of the sun.
Step-by-step explanation:
The flattened appearance of the sun when it is just before setting is due to atmospheric refraction and scattering. The Earth's atmosphere bends the light from the sun, causing it to appear higher above the horizon than it actually is. This bending of light also causes the sun to appear flattened due to the way we perceive its shape through the atmosphere.
Additionally, the longer path that sunlight must travel through the atmosphere during sunset increases the chance for sunlight to be scattered. Since red light is less likely to be scattered than blue light, the sun appears more reddish as it approaches the horizon, adding to the perception of a flattened shape.
Therefore, the correct answer is D) The observer's position on Earth affects the visual perception of the sun.