What is the answer of this Q?

Because of refraction.

Explanation:

Light coming from fish passes through two medium, one is water and another is air. As the density of medium is changing from high to low, the light bends away from the normal. So to the human eyes, the fish appears to be above its actual position.

Light rays reflected from the fish are refracted at the surface of the water, but the eyes and brain trace the light rays back into the water as thought they had not refracted, but traveled away from the fish in a straight line. This effect creates a "virtual" image of the fish that appears at a shallower depth.

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