## Core Concept
Circular DNA is a characteristic feature of certain organelles found in eukaryotic cells. The primary sites of circular DNA in eukaryotes are mitochondria and chloroplasts. These organelles are believed to have originated from ancient bacteria, which had circular DNA.

## Why the Correct Answer is Right
The correct answer, **D. Nucleus**, is right because the nucleus, which is the primary organelle found in eukaryotic cells, contains linear DNA, not circular DNA. The nuclear DNA in eukaryotes is organized into multiple linear chromosomes.

## Why Each Wrong Option is Incorrect
- **Option A:** Mitochondria do contain circular DNA, known as mtDNA, which encodes for some of the proteins involved in oxidative phosphorylation.
- **Option B:** Chloroplasts, found in plant cells and some algae, also contain circular DNA, known as cpDNA, which is involved in encoding some of the genes necessary for photosynthesis.
- **Option C:** While not as commonly referenced as mitochondria or chloroplasts for circular DNA, certain types of viruses can infect eukaryotic cells and have circular DNA. However, the context here seems to focus on organelles within eukaryotic cells.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **mitochondrial DNA** is circular and solely inherited from one's mother, as only egg cells contribute mitochondria to the zygote. This has significant implications for genetic diseases related to mitochondrial function.

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