The correct answer is Nucleolus.

- The nucleolus is a distinct structure found within the nucleus of eukaryotic cells.
- It plays a crucial role in the synthesis and assembly of ribosomal subunits.
- Specifically, the nucleolus is responsible for the transcription of ribosomal RNA (rRNA) genes, processing of the rRNA, and combining it with ribosomal proteins imported from the cytoplasm to form the small and large ribosomal subunits.
- These subunits are then transported out of the nucleus to the cytoplasm where they assemble into functional ribosomes, essential for protein synthesis.

In contrast, other cellular structures have different primary functions:
- The Golgi apparatus is involved in modifying, sorting, and packaging proteins and lipids for secretion or use within the cell.
- The endoplasmic reticulum (ER), especially the rough ER, is the site of synthesis for membrane-bound and secretory proteins but does not assemble ribosomal subunits.
- The mitochondrion is primarily responsible for energy production through ATP synthesis and has its own small ribosomes, but it does not contribute to the biogenesis of the cell's main ribosomal subunits.

Reference: Molecular Biology of the Cell, 6th Edition, Chapter 6: The Cell Nucleus and Chromosomes