Light travels slower in glass than in air because the refractive index of air is less than the refractive index of glass (option A). The refractive index is a measure of how much light bends when it enters a material. Air, having a lower refractive index, allows light to travel through it with minimal interaction and minimal slowing down. In contrast, glass has a higher refractive index, meaning light encounters more resistance as it passes through. This resistance occurs because light interacts more with the atoms and molecules in the glass, which absorb and re-emit the light waves, effectively slowing their overall speed. While the density of the materials can influence their refractive properties, the primary reason for the difference in light speed is the refractive index, making light travel slower in glass than in air.