Answer
(d) increase the index of refraction of the medium in which the setup is immersed
Work Step by Step
Correct, increasing the index of refraction of the medium in which the setup is immersed will cause the separation between adjacent minima on the screen to decrease. This is because the index of refraction affects the phase velocity of light, and as the index of refraction increases, the phase velocity also increases. This causes the path difference between the light passing through the two slits to be reduced, which in turn causes the constructive and destructive interference to occur at closer spacings on the screen.
