Final answer:
The chromosome regions containing rDNA, which are crucial for ribosomal synthesis, are located in the nucleoli of eukaryotic cells.
Step-by-step explanation:
The chromosome regions containing rDNA are located in the nucleoli. Inside the nucleus of eukaryotic cells, chromosomes are made up of strands of DNA wrapped around proteins and contain genes, which include sections of DNA that encode for ribosomal RNA (rRNA). The nucleolus aggregates the rRNA with associated proteins to assemble ribosomal subunits. These subunits are vital for protein synthesis, and they are transported out of the nucleus into the cytoplasm to complete their assembly into functional ribosomes.