Answer: 10 cm
Step-by-step explanation:
The water at 10 cm has the biggest survival rate for the embryos with ultraviolet B protection as compared to that of embryo which are not protected by UV-B.
Ultraviolet rays can be very harmful as it can reach the fetus but due to some particles present in water the intensity of UV-B becomes attenuated below 10 cm of water.
So, the correct answer is 10 cm.