Answer:
b. 6280 cubic feet
Explanation:
Volume:
The volume of the prism, which has a cylindrical base, with radius r and height h, is given by:

Radius:
The radius is 20, so

The manufacture recommends that it be filled to within 1 foot of the rim.
Height of 6 - 1 = 5, so

Then

So the volume is 6280 cubic feet, and the correct answer is given by option b.