In which of these situations would light slow down?

a. from plastic to air
b. from ice to water
c. from a vacuum to air
d. from glass to air

Option (C) from a vacuum to air

Explanation:

Speed of light in any media depends on the refractive index(n) of the medium in which light is travelling.The medium having least refractive index have the fastest speed of the light.As the refractive index of the vacuum is less(1.0000) than that of the air(1.0003), so the speed of light will slows down while moving from vacuum to air.The refractive index 'n' is the ratio between speed of light in vacuum(c) to the speed at which the light is travelling i.e v,  (n = c / v).