The optical phenomenon responsible for the blue color of the sky is scattering, specifically Rayleigh scattering.

Option 1, dispersion, is the phenomenon where light splits into its constituent colors, as seen in a rainbow, and does not explain why the sky appears blue.

Option 2, reflection, is when light bounces back from a surface, which also doesn`t explain the blue sky as the sky isn`t a solid surface from which light can reflect.

Option 3, refractions, which is the bending of light as it passes from one transparent medium to another would also not lead to a blue sky, but rather how we view images through a transparent medium.

Option 4, scattering, is correct. Rayleigh scattering occurs when the particles on the path of light are much smaller than the wavelength of the light. Shorter wavelengths (blue/violet) are scattered more than the longer ones (red, yellow, green). However, we perceive the sky as blue instead of violet because our eyes are more sensitive to blue light and because sunlight reaches us more in the blue part of the spectrum, rather than violet.