Which is the phenomenon responsible for dispersion of light?

Refraction is the correct reason responsible for the dispersion of light. Dispersion of light is defined as the phenomena of splitting up of composite light into its constituent colours.

The refraction is the phenomena in which light is deviated from its original path due to the difference in refractive indexes or we can say difference in densities of the medium through which light is travelling.

Now each part of the composite light deviate differently according to its wavelength. The deviation of light from its original path is called refraction and the refraction of different component of composite light differently, causes dispersion of light.

While there is no role of reflection, tyndal effect and scattering here. So, all other options are wrong.