Question

what is the index of refraction?
the difference between the angle at which the light ray enters the new medium and the angle at which it is reflected
the ratio of the speed of light in a vacuum to the speed of light in the material it is passing though
the amount the light ray has bent toward the normal line when entering a new medium
the percentage that the light ray bent when it entered the new medium

Explanation:

The index of refraction (refractive index) is a fundamental property of a transparent material, defined by the ratio of the speed of light in a vacuum to the speed of light within that material. The other options describe refraction angles or bending, not the index itself.

Answer:

the ratio of the speed of light in a vacuum to the speed of light in the material it is passing though