Explanation:

- Alpenglow (Option 3): This is the correct answer. Alpenglow refers to the rosy or orange hue seen on mountain tops around sunrise and sunset. It occurs due to the scattering of sunlight by the atmosphere, which illuminates the mountains even when the sun is below the horizon.

- Brillouin scattering (Option 1): This is related to the scattering of light by sound waves or density variations inside a material, not atmospheric phenomena.

- Circle of confusion (Option 2): This term is used in photography to describe the size of the blur spot caused by a point source that is not in focus.

- Barrel distortion (Option 4): This is a type of distortion where image magnification decreases with distance from the optical axis, making images appear barrel-shaped.