On devices like your smartphone, there is a small camera. Your smartphone is very thin, so the lens must be very small and the distance between the lens and screen of this camera is very short. To make sure you can create images on this screen a short distance away, the lens

a) must have a short focal distance, so the index of refraction should be a high value.

b) must have a long focal distance, so the index of refraction should be a high value.

c) must have a short focal distance, so the index of refraction should be a low value.

d) must have a long focal distance, so the index of refraction should be a low value.