I believe that the answer is D.
If each bag covers 1.25 ft^2 then you need to divide the total area 60 ft^2 by 1.25 ft^2 and that will tell you the number of bags needed which is 48 bags
20m measurement that’s what is equal to 10
The volume of the triangular prism is the Area of the base times the height
the area of the base is:

and the volume is:

now b=10 yards, a=20 yards, and V=600 yards^3
we clean the height

the answer is height = 6 yards