Final answer:
The correct answer is B. Nucleolus, which is the part of the cell's nucleus responsible for producing and assembling ribosomes, a process critical for protein synthesis.
Step-by-step explanation:
The correct answer is B. Nucleolus. The nucleolus is a small, dense mass within the nucleus of a cell. In contrast to other options which are related to bone cells and tissues, the nucleolus is associated with the production of ribosomes in eukaryotic cells and does not have a role in bone formation or maintenance. Specifically, the nucleolus is responsible for synthesizing ribosomal RNA (rRNA) and assembling ribosomes, which are essential for protein synthesis.
The term nucleus refers to different structures in different contexts: in cells, it is the organelle that contains DNA; in atoms, it is the core containing protons and neutrons; and, in the brain, it is a center of nerve function. The other terms given in the options pertain to bone cells: osteoclasts break down bone tissue, osteocytes are mature bone cells, and periosteum is the membrane that covers bones. Thus, for a small mass within the nucleus of a cell, nucleolus is the appropriate answer.