Answer:
air or vacuum is the least refractive index medium of most rare medium
all other medium has more value of refractive index and hence the speed will be less than the speed of the air
Step-by-step explanation:
As we know that speed of light in any medium and speed of light in air is related to each other by the equation

here we know that

[tex]\mu [tex] = refractive index of the medium
so we know that air or vacuum is the least refractive index medium of most rare medium
all other medium has more value of refractive index and hence the speed will be less than the speed of the air