A medium in which the speed of light is less is called

A medium is said to be optically rarer if light moves through it more quickly. An optically transparent media is one that allows light to travel through it without any difficulty or impediment. An optically denser medium is one in which the speed of light is lower than usual.
Due to the fact that light travels at various speeds depending on the mphenomenonaterial it is travelling through, we experience a phenomena known as refraction of light. the shift in light's direction when it moves from one medium to another. Light refraction is the term used to describe this phenomena.

The light beam encounters a shift in its course as it travels through one medium and then another. The incident ray alters its route here in the diagram, travelling from the air to the glass and then back to the air after passing through the glass slab.
Depending on how fast light travels in a certain medium compared to other media, we may divide it into two categories.
1. An optically rarer medium has a higher light speed.
2. Optically denser media: The speed of light is slower in this medium.
The solution is "optically denser medium," as a result.

Hence, the correct option is C.