Final answer:
The structure responsible for making ribosomes in the cell is the nucleolus, found within the nucleus. It synthesizes rRNA and initiates the assembly of ribosome subunits, which are vital for protein synthesis.
Step-by-step explanation:
The cellular structure responsible for making ribosomes in the cell is the nucleolus. The nucleolus is located within the nucleus of eukaryotic cells and it is dedicated to the assembly of ribosome components. In this cellular 'factory', ribosomal RNA (rRNA) is synthesized and combined with proteins to form the large and small subunits of ribosomes. These subunits then exit the nucleus to the cytoplasm where they fully assemble into functional ribosomes. Ribosomes are essential for protein synthesis as they read the mRNA sequence and translate it into the corresponding amino acid sequence of a protein.
Ribosomes exist either as floating complexes in the cytoplasm or attached to the cytoplasmic side of the endoplasmic reticulum, forming the rough ER, which further emphasizes their central role in protein synthesis. Therefore, the correct answer to the question is C) Nucleolus, which aligns with answer d. nucleolus from the provided reference information.