Answer:
a). Vacuum
Step-by-step explanation:
As we know that the refractive index of the medium is defined as

here we know that
c = speed of light in vacuum
v = speed of light in medium
so here we know that

so speed will be maximum for that medium for which value of refractive index is minimum
So here from all given options minimum refractive index is for vacuum
so correct answer will be
a). Vacuum