The speed of light v in a medium of refractive index n is given by:
where c is the speed of light in vacuum. Now, each colour has different refractive index in a medium. Thus, they have different speeds (except in vacuum). As the relation between speed and refractive index is inverse, the first colour to emerge should have the lowest refractive index. Red has the lowest refractive index. Hence, option (d) is correct.