**Core Concept:** RNA stands for Ribonucleic Acid, which is a nucleic acid molecule that plays a crucial role in encoding, storing, and transmitting genetic information within cells. There are three main types of RNA, namely messenger RNA (mRNA), ribosomal RNA (rRNA), and small nuclear RNA (snRNA).

**Why the Correct Answer is Right:** The correct answer, **Option D: Ribosomes**, is related to ribosomal RNA (rRNA). Ribosomes are tiny, protein-RNA complexes found in the cytoplasm of eukaryotic cells and within the stroma of prokaryotic cells. They are responsible for protein synthesis, which occurs during transcription and translation processes. The correct answer describes the cellular location where rRNA is found, specifically within ribosomes.

**Why Each Wrong Option is Incorrect:**

A. **Mitochondria (Option A)**: While mitochondria are organelles in eukaryotic cells involved in energy production, they do not contain RNA. Instead, they contain their own DNA (mitochondrial DNA) and require nuclear-encoded proteins for their functions, which are synthesized in the cytoplasm by ribosomes.

B. **Nucleus (Option B)**: The nucleus is the organelle in eukaryotic cells where DNA is housed. However, RNA is synthesized and processed in the nucleus, primarily mRNA, but also other RNA types like snRNA and snoRNA. Ribosomes are not found within the nucleus.

C. **Cytoplasm (Option C)**: While RNA is found in the cytoplasm, ribosomes, containing rRNA, are located within the cytoplasm of eukaryotic cells and within prokaryotic cells' stroma.

D. **Ribosomes (Option D)**: As explained earlier, ribosomes are the cellular site where proteins are synthesized from RNA. In eukaryotic cells, ribosomes are found in the cytoplasm, while in prokaryotic cells, they are located within the stroma.

**Clinical Pearl:** The functional unit of ribosomes is the ribosome subunit, which is composed of small and large ribosomal RNA (rRNA) and ribosomal proteins. These subunits come together to form the complete ribosome, where translation takes place during protein synthesis. Understanding the cellular location of RNA and ribosomes is essential for understanding cellular processes like transcription, translation, and protein synthesis.