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 preāribosomal ribonucleic acid (rRNA). So, the nucleolus is the place of rRNA transcription, pre-rRNA processing and ribosome subunit assembly.