An area within the nucleus that appears through the microscope due to the densely stained chromatin fibers where ribosomal rna is being produced in high quantities.

The correct answer is nucleolus.

The nucleolus is the structure inside the nucleus of the eukaryotic cell where ribosome biogenesis occurs. assembles ribosomal subunits in eukaryotic cells. Foundation for the nucleolar structure are the genes for pre-ribosomal ribonucleic acid (rRNA). So, the nucleolus is the place of rRNA transcription, pre-rRNA processing and ribosome subunit assembly.