The answer is d) 20 m.
The depth of pond is 20 m if it seems to be 10 m.
The formula relating real depth and app. depth :
In this case, refractive index has to be an whole number for real depth to give a whole number for app. depth.
- Then, real depth must be a multiple of 10, but refractive index cannot be 1
- The option is 20 m