Final answer:
The index of refraction of a material is a property that describes how much the speed of light is reduced when it travels through the material compared to its speed in a vacuum. In this case, the index of refraction of the material is 4.
Step-by-step explanation:
The index of refraction, denoted by 'n', is a property of a material that describes how much the speed of light is reduced when it travels through the material compared to its speed in a vacuum. The index of refraction is given by the equation n = c/v, where c is the speed of light in a vacuum and v is the speed of light in the material.
In this case, the speed of light in the transparent medium is 2/8 the speed of light in a vacuum. Since the index of refraction is the ratio of the speed of light in a vacuum to the speed of light in the material, the index of refraction of the material would be 8/2 = 4.
Therefore, the correct answer is d. 4.