**Core Concept**
Eukaryotes are a domain of organisms characterized by the presence of a true nucleus enclosed within a nuclear envelope, distinct from prokaryotes. Eukaryotic cells have complex organelles and structures, including mitochondria, chloroplasts, and a cytoskeleton.

**Why the Correct Answer is Right**
Mitochondria are a key feature of eukaryotic cells, responsible for generating energy through cellular respiration. They have their own DNA and are thought to have originated from a symbiotic relationship between ancient bacteria and the early eukaryotic cells. This unique feature distinguishes eukaryotes from prokaryotes, which lack membrane-bound organelles.

**Why Each Wrong Option is Incorrect**
**Option A:** Bacteria are prokaryotes, characterized by the absence of a true nucleus and membrane-bound organelles.
**Option B:** Viruses are not considered living cells, but rather infectious agents that replicate within host cells, and do not possess cellular structures like mitochondria.
**Option C:** Archaea are prokaryotes, similar to bacteria, but with distinct metabolic and structural characteristics.

**Clinical Pearl / High-Yield Fact**
Eukaryotic cells, including human cells, rely on mitochondria for energy production through oxidative phosphorylation, which is essential for maintaining cellular homeostasis and function.

**Correct Answer: C. Mitochondria.**