The answer to this question would be: A. Water
The speed of light will be decreased when it moves in a media that has a higher index of refraction. In the atmosphere, the light is move inside air. When light moves inside water, it will be slower than when it moves in the atmosphere because water index of refraction is 1.33 times of air.
Pencil looks like bent when you put on water is an example of what happens by the difference of index of refraction.
