**Core Concept**
Mitochondria are the primary sites of cellular respiration, responsible for generating energy in the form of ATP through the process of oxidative phosphorylation. This process involves the transfer of electrons through a series of electron transport chains, ultimately driving the production of ATP.

**Why the Correct Answer is Right**
The correct answer is related to the structure and function of mitochondria. Mitochondria have a unique double membrane structure, with the inner membrane being folded into cristae, which increases the surface area for electron transport. This unique structure allows mitochondria to efficiently generate ATP. The electron transport chains are embedded in the inner mitochondrial membrane and consist of a series of protein complexes that transfer electrons from high-energy molecules to oxygen, generating a proton gradient that drives ATP synthesis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Mitochondria have a single membrane. This is incorrect because mitochondria have a unique double membrane structure.
* **Option B:** Mitochondria are the site of glycolysis. This is incorrect because glycolysis occurs in the cytosol of the cell, not in the mitochondria.
* **Option C:** Mitochondria have a role in protein synthesis. This is incorrect because mitochondria have their own ribosomes and can synthesize some of their own proteins, but they do not have a significant role in protein synthesis in the rest of the cell.

**Clinical Pearl / High-Yield Fact**
It's worth noting that mitochondria have their own DNA, known as mtDNA, which is separate from the nuclear DNA found in the cell's nucleus. This mtDNA is responsible for encoding some of the proteins involved in the electron transport chains.

**Correct Answer: D. Mitochondria have a unique double membrane structure.**