Air. The refractive index of air is approximately 1, and the velocity of light in a medium is given by v = c/n, where c is the speed of light in vacuum and n is the refractive index of the material. Out of the materials listed, air has the smallest refractive index hence velocity is highest.
In which of the following materials is the velocity of light greatest a. Water B. Fused quartz C. Air D. Crown d. glass