Which cellular structure is primarily responsible for the synthesis of ribosomal RNA?

The nucleolus is the cellular structure primarily responsible for the synthesis of ribosomal RNA (rRNA). It is located within the nucleus and serves as the site where rRNA is transcribed and combined with proteins to form the subunits of ribosomes. The nucleolus is not just a passive structure; it actively assembles the components needed for ribosome production, which is crucial for protein synthesis in the cell.

While the nucleus contains the genetic material and is involved in various aspects of gene expression, it is the nucleolus specifically that focuses on rRNA synthesis and ribosome assembly. Other structures like ribosomes are the sites of protein synthesis but do not produce rRNA. The endoplasmic reticulum, particularly the rough variety, is involved in the synthesis of proteins but not in the synthesis of ribosomal RNA itself. Therefore, the nucleolus stands out as the dedicated site for rRNA synthesis, making it the correct answer.