The correct response is:
C The index is inversely proportional to the speed of light in the medium.
This is because the index of refraction (n) is defined as the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v):
\[ n = \frac{c}{v} \]
As the speed of light in the medium decreases (lower v), the index of refraction increases (higher n), showing that they are inversely related.