The statement that it causes light to slow down significantly best describes the high index of refraction.
<u>Explanation:</u>
Index of refraction is the amount a medium will be refracting the incident light. As it is a measure of amount of light getting refracted, the medium decides the index of refraction of that type.
If the index of refraction of any medium is high, then it will refract most of the incident light and so the light' speed travelling through the medium will get slower.
So increase in refractive index decrease the light' speed. Thus, if the light' speed decreases or slows down significantly then the substance has high index of refraction.
