## Core Concept
The question tests the understanding of cellular biology, specifically the distribution and function of DNA and RNA within cells. DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are both crucial nucleic acids found in cells, with DNA primarily storing genetic information and RNA involved in protein synthesis and other cellular processes.

## Why the Correct Answer is Right
The correct answer, , refers to a structure that contains both DNA and RNA. In cellular biology, the **nucleus** contains DNA, while **ribosomes**, which are found throughout the cytoplasm and also attached to the endoplasmic reticulum, contain RNA. However, a specific organelle known as the **mitochondrion** contains its own DNA (mitochondrial DNA) and also has ribosomes for protein synthesis, which are made of RNA. Therefore, mitochondria uniquely contain both DNA and RNA.

## Why Each Wrong Option is Incorrect
- **Option A:** typically refers to structures or organelles that primarily contain DNA, such as the nucleus, but not both DNA and RNA.
- **Option B:** could refer to various organelles, but without specificity, it's hard to directly refute. However, most organelles are specialized to contain one or the other, not both.
- **Option C:** might imply structures with RNA, like ribosomes, but these do not contain DNA.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **mitochondria** are often referred to as the "powerhouses" of the cell and have their own DNA (mtDNA), which is separate from the DNA found in the nucleus. This mtDNA is crucial for encoding some of the proteins involved in oxidative phosphorylation. This fact is particularly relevant for understanding mitochondrial diseases, which can result from mutations in either the mitochondrial DNA or the nuclear DNA that encodes mitochondrial proteins.

## Correct Answer Line
**Correct Answer: D. Mitochondria**