Final Answer:
The dense mass within the nucleus containing ribonucleic acid is referred to as Nucleolus. Option B is the answer.
Step-by-step explanation:
The dense mass within the nucleus containing ribonucleic acid (RNA) is called the nucleolus. This cellular structure is responsible for the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomal subunits. The nucleolus plays a crucial role in the production of ribosomes, essential for protein synthesis in cells. It is distinct within the nucleus and can often be visualized using microscopy techniques.
The other options do not accurately describe the structure responsible for RNA synthesis and ribosome assembly within the nucleus.
Option B is the answer.