Solution:
Concept: Because light travels slower in water than in air, the image we see when we fill a tank with water and look down will be an apparent image.
Let it be filled to height 'h' cm. Then the part not in water is (21-h) cm. So the apparent depth of The filled portion must be (21-h) cm.
refractive index of water
So, 
water should be filled in a container.Hence, option 1 is correct.
