Step 1: Recall the site of rRNA synthesis.
Ribosomal RNA (rRNA) is synthesized in the
\[
\boxed{\text{Nucleolus}}
\]
by RNA polymerase I.
The nucleolus is also the site of ribosome assembly.
Step 2: Choose the correct option.
Hence, the biosynthesis of ribosomal RNA occurs in
\[
\boxed{\text{Nucleolus}.}
\]
Therefore, the correct option is \(\boxed{(D)}\).