## **Core Concept**
Ribosomal RNA (rRNA) is a crucial component of ribosomes, which are responsible for protein synthesis in cells. The production of rRNA is essential for the synthesis of ribosomes. This process primarily occurs in a specific region of the cell.

## **Why the Correct Answer is Right**
The correct answer, **nucleolus**, is the site within the nucleus where rRNA synthesis and ribosome assembly occur. The nucleolus is rich in RNA and proteins and is the location where the genes for rRNA are transcribed by RNA polymerase I. The rRNA transcripts then undergo processing and are assembled with proteins to form ribosomal subunits.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **cytoplasm** is not the primary site for rRNA production. While ribosomes are found in the cytoplasm and rRNA is a component of ribosomes, the actual synthesis of rRNA occurs in the nucleus, specifically in the nucleolus.
- **Option B:** The **mitochondria** do contain their own ribosomes and can synthesize some of their own rRNA, but this is not the main site for rRNA production for the cell's cytoplasmic ribosomes.
- **Option D:** The **cell membrane** is not involved in rRNA production. It is a structure that encloses the cell and regulates the movement of materials in and out.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **nucleolus** is often prominent in cells that are actively growing and dividing, such as cancer cells, because these cells require a high rate of protein synthesis. This can sometimes be used as a diagnostic feature in pathology.

## **Correct Answer:** C. Nucleolus.