Refraction occurs due to the bending of light rays as they pass from one medium to another. The light rays bend as their speed changes as they pass from one medium to another. The more the speed changes, the more the light bends and therefore more refraction occurs. When light passes from air to the diamond, the speed of light decreases more than any of the other materials listed in the given choices. So the light rays slow down the most diamond and so most refraction occurs in diamond.
