The correct answer is option 1 - refractive index of air is less than that of glass.

When light travels through a medium, its speed is dependent on the refractive index of that medium. The refractive index is a measure of how light bends or slows down when it passes through a substance.

In this case, when light enters glass from air, it slows down because the refractive index of glass is higher than that of air. The refractive index of a medium is a measure of how much the speed of light is reduced in that medium compared to a vacuum. Since the refractive index of glass is higher than air, it means that light travels at a slower speed in glass than in air.

Options 2 and 3 are incorrect because they suggest that the refractive index or density of air is greater than that of glass, which is not the case.

Option 4 is also incorrect because it suggests that the density of glass is less than that of air, which is not true. The density of glass is actually greater than that of air.

Therefore, the correct explanation for why light travels slower in glass than in air is that the refractive index of air is less than that of glass.