Ribosomes are produced and assembled in

The correct option is "nucleolus".

Step-by-Step Explanation:

1. Understanding Ribosomes: Ribosomes are essential cellular structures responsible for protein synthesis (translation) in all living cells. They consist of ribosomal RNA (rRNA) and ribosomal proteins.
2. The Role of the Nucleolus: The nucleolus is a dense, distinct sub-structure located inside the nucleus of eukaryotic cells. It is not membrane-bound but functions specifically as the site for transcribing ribosomal RNA genes into rRNA molecules.
3. Assembly Process: Ribosomal proteins synthesized in the cytoplasm are imported into the nucleus and enter the nucleolus. Inside the nucleolus, these proteins associate with the newly transcribed rRNA molecules to form the individual small and large ribosomal subunits.
4. Export to Cytoplasm: Once the ribosomal subunits are assembled in the nucleolus, they are exported out of the nucleus through nuclear pores into the cytoplasm, where they fully assemble into functional ribosomes during translation.

Therefore, the nucleolus is the specific site where ribosomes are produced and assembled.