Answer: Option B: the ratio of the velocity of light in a vacuum over the velocity of light in the medium, n = c/v
Explanation:
When a wave of light enters in some materials, the velocity changes depending on the material, and this is why some times when light enters in something, for example, a glass of water, the "path" of the light changes (and you can see some cool visual effects)
then, we define the refractive index of a medium as:
n = c/v
where n is the refractive index, c is the velocity of the light in the vacuum and v is the velocity of the light in the material, here you can see that n is always greater or equal than 1 ( in the case n = 1, we also have v= c)
Then, the correct option is:
option B: the ratio of the velocity of light in a vacuum over the velocity of light in the medium
