Answer:
 70686 sq.ft.
Step-by-step explanation:
Given : Radius of the whole space is 250 ft.
             Radius of pond is 200 ft.
To Find:  the square feet of gravel needed
Solution:
Area of whole space = 
                                   = 
                                   = 
Area of Pond = 
                                   = 
                                   = 
So, the square feet of gravel needed =
                                                               =
Hence 70650 sq.ft. of gravel is needed.
So, Option D is approximately equal to the obtained answer
So, Option D is true.