Answer is : (b) 160 m2
Girl can observe only those light rays which are refracted and leaves the glass slab at angle of 90° or less as shown below.
Now, from Snell’s law in layer 1 and 3, we have
Now, from Pythagoras theorem, we have
⇒ tan r = 3/√7
So, from figure, we have
Hence, area of pool visible through glass slab is A = πd2/4 = \(\frac{\pi\times(2x+0.6)^2}{4}\) ≈ 160 m2