Final answer:
The site within the eukaryotic nucleus where ribosomal subunits are assembled is called the nucleolus.
Step-by-step explanation:
In eukaryotes, the assembly of ribosomal subunits occurs in a site within the nucleus called the nucleolus. The nucleolus aggregates ribosomal RNA (rRNA) with associated proteins to assemble the two ribosomal subunits, which are then transported out through the pores in the nuclear envelope to the cytoplasm where they participate in protein synthesis. Ribosomes in eukaryotic cells consist of a large and a small subunit, and these complexes are essential for the translation of mRNA into proteins.