As diameter is twice the radius, if the diameter of the base of the cylinder is 20 feet, its radius is 10 feet $$=120$$ inches ( $$=r$$ say ) ( 1 feet $$=12$$ inches ).

The level/height at which the cylinder to be filled is 3 feet 6 inches $$=3 \times 12+6=36+6=42$$ inches ( $$=\mathrm{h}$$ say).

Note: - Though the original height of the cylinder is 4 feet, but as the cylinder is to be filled up to 3 feet 6 inch height, the height while calculating the volume would be considered as 3 feet 6 inches only.

So, the volume of water in cylinder is $$\pi \mathrm{r}^2 \mathrm{~h}=\frac{22}{7} \times(120)^2 \times 42=1900800$$ cubic inches which is same as $$\frac{1900800}{231}=\frac{57600}{7}$$ gallon (Using 1 gallon $$=231$$ cubic inches)

Hence the answer is (B).