D is correct. This has to do with the frequency dependence of the index of refraction. This effect can be seen in prisms and is what allows them to break white light into its different colors (frequencies). If they all bent the same amount, the beam would stay together and the light leaving a prism would remain white. Also rainbows would not be possible, since they are made by light refracting at different angles inside spherical droplets of water in the earth's atmosphere.
