Here's why:

* Nucleolus: The nucleolus is a prominent, spherical structure found within the nucleus of eukaryotic cells. It is the site of ribosome biogenesis, where ribosomal RNA (rRNA) is transcribed and assembled with ribosomal proteins to form ribosomal subunits.

* RNA: The nucleolus is rich in RNA, primarily rRNA, which is essential for the formation of ribosomes.

* Dense: The nucleolus appears dense because it is packed with rRNA, ribosomal proteins, and other factors involved in ribosome assembly.

Therefore, the nucleolus is the correct answer to your question.