Bottom of a tank containing water appears to be raised. It is due to

When light rays travel from a denser medium (water) to a rarer medium (air), they bend away from the normal at the interface. This phenomenon is known as refraction of light.

Due to this bending, the virtual image of the bottom of the tank is formed above the actual bottom, making the apparent depth less than the real depth. Hence, the bottom of the tank appears to be raised.

Answer: Refraction of light