Answer
(d) all of the preceding.
Work Step by Step
The index of refraction $n= \frac{c}{v}=\frac{wavelength\,\, in \,\,vacuum}{wavelength\,\, in\,\, the\,\, medium}$
Therefore, since v is always less than c in any medium, n is always greater than or equal to 1. Also, the index is inversely proportional the speed of light in the medium and also to the wavelength of light in the medium.
So, (d) all of the preceding is correct.
