The correct option is A Velocity of light differs from one medium to another
It's because of a phenomena called
refraction. When a light beam travels from one medium to another, then
it's velocity decreases or increases depending upon the optical density
of the other medium. Notice the word beam here. A beam is a collection
of many light rays. All light rays in a beam move individually. In
reality we only have a narrow beam of light, a ray of light is idealization. When a light beam enters another medium at an slanting
angle the ray that reaches the medium first changes it's velocity before
any other ray, the change in velocity causes the beam to bend.