What specific structures within the nucleus help in the formation of ribosomes?

The formation of ribosomes takes place in the nucleolus, which is a specialized sub-structure within the nucleus of eukaryotic cells. The nucleolus is primarily responsible for the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomal subunits, which are critical components of ribosomes. These ribosomal subunits then exit the nucleus through the nuclear pores into the cytoplasm, where they combine with messenger RNA (mRNA) to function in protein synthesis.

Other structures mentioned in the options, such as mitochondria, chromosomes, and nuclear pores, do not play a direct role in ribosome formation. Mitochondria are involved in energy production, chromosomes contain the genetic material, and nuclear pores function as gateways for the transport of molecules between the nucleus and the cytoplasm, but they do not contribute to the actual assembly of ribosomes. Thus, the nucleoli are specifically designated for the critical task of ribosome biogenesis, making it the correct answer to the question.