The correct answer is:
b) n=f2/f1
Index of refraction (n) is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in a particular medium (v): n = c/v. By using the relation v = fλ, where f is the frequency and λ is the wavelength of light in the medium, we can relate the index of refraction to the ratios of frequencies.