The correct option is B Refraction
The rays of light coming from the bottom of the swimming pool travel from water to air. As the rays pass from denser medium (water) to rarer medium (air), they bend away from the normal. When the rays are produced back, they form an image of the bottom of the pool at a point above the real position. Therefore, the swimming pool appears to be less deep that it actually is.
This phenomenon takes place due to the refraction of light at the interface of the two media.
Hence, the correct answer is OPTION B.