Aray of light passes from air into water with an angle of incidence of 30 degrees.

which of the following quantities does not change as the light enters the water?

a. wavelength
b. frequency
c. speed of propagation

This described situation is known as Refraction.

Refraction is a phenomenon in which the light  bends or changes it direction when passing through a medium with a refractive index different from the other medium.  

Being a relation between the speed of light in vacuum and its speed in the other medium. This means the speed of propagation does change.

The wavelength may be modified because it depends on the medium.

Nevertheless, in this process the refracted ray of light does not change its frequency , because  frequency is:

Where is the period of the wave and this remains unchanged, hence the frequency, as well.

it doesn't. it was imagined up in the middle ages.