Concept:
For two parallel pipes connecting the same reservoirs,
the head loss in each pipe is the same.
Using Darcy-Weisbach equation,
\[
h_f\propto\frac{Q^2}{D^5}
\]
Since the head losses are equal,
\[
\frac{Q_1^2}{D_1^5}
=
\frac{Q_2^2}{D_2^5}.
\]
Therefore,
\[
\boxed{
\frac{Q_1}{Q_2}
=
\left(\frac{D_1}{D_2}\right)^{5/2}
}
\]
Step 1: Write the given diameters.
\[
D_1=2d,
\]
\[
D_2=d.
\]
Step 2: Calculate the discharge ratio.
\[
\frac{Q_1}{Q_2}
=
\left(\frac{2d}{d}\right)^{5/2}
=
2^{5/2}
=
4\sqrt2.
\]
Hence,
\[
\boxed{
Q_1:Q_2
=
4\sqrt2:1.
}
\]
Therefore, the correct option is
\[
\boxed{(D)\;4\sqrt2:1.}
\]