**Core Concept**
Mammalian mitochondria play a crucial role in various cellular processes, primarily energy production, but also other metabolic pathways. The mitochondria contain their own DNA (mtDNA) and are involved in the synthesis of certain mitochondrial proteins, as well as the regulation of other cellular processes.

**Why the Correct Answer is Right**
Mitochondria are indeed responsible for fatty acid oxidation (β-oxidation), which breaks down fatty acids to produce acetyl-CoA, a key molecule in the citric acid cycle. This process occurs in the mitochondrial matrix. Furthermore, mitochondria are involved in DNA synthesis, as they contain mtDNA and are responsible for the replication and repair of their own genetic material. Fatty acid synthesis, on the other hand, primarily occurs in the cytosol, where acetyl-CoA is converted into fatty acids through a series of reactions.

**Why Each Wrong Option is Incorrect**
**Option A:** Fatty acid synthesis primarily occurs in the cytosol, not in the mitochondria, making this option incorrect.
**Option B:** Mitochondria are indeed involved in DNA synthesis, specifically the replication and repair of mtDNA, so this option is incorrect.
**Option C:** Mitochondria are responsible for fatty acid oxidation (β-oxidation), making this option incorrect.

**Clinical Pearl / High-Yield Fact**
Mitochondria have their own genetic material, mtDNA, which is separate from the nuclear DNA found in the cell's nucleus. This unique aspect of mitochondria is crucial for their function and regulation.

**Correct Answer Line**
✓ Correct Answer: D. Protein synthesis