well, we know the diameter of the cylinder is 42, thus its radius is half that, or 21 feet. Since we know its radius and height, 7 feet deep, we can get the exact volume of ft³ it takes.
[tex]\bf \textit{volume of a cylinder}\\\\
V=\pi r^2 h~~
\begin{cases}
r=radius\\
h=height\\
-----\\
r=21\\
h=7
\end{cases}\implies V=\pi (21^2)(7)[/tex]
now, compare that with any of those estimates, keeping in mind that they're just approximations.
