**Core Concept**
Atherosclerotic aneurysms are characterized by the weakening of arterial walls due to the accumulation of lipids, inflammatory cells, and fibrotic tissue, leading to the formation of an aneurysm. This process is primarily driven by chronic inflammation and damage to the arterial wall, particularly affecting the aorta and its major branches.

**Why the Correct Answer is Right**
Atherosclerotic aneurysms most commonly occur in the abdominal aorta, particularly at the level of the visceral arteries. This is due to the high concentration of atherosclerotic plaques and the subsequent weakening of the arterial wall. The abdominal aorta is more susceptible to atherosclerotic changes due to increased wall stress, turbulent blood flow, and the presence of branch points, which can create areas of high shear stress.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while the thoracic aorta can be affected by atherosclerosis, it is less common than the abdominal aorta for atherosclerotic aneurysm formation.
* **Option B:** This option is incorrect because while the popliteal artery can be affected by atherosclerosis, it is not the most common site for atherosclerotic aneurysm formation.
* **Option D:** This option is incorrect because while the femoral artery can be affected by atherosclerosis, it is not the most common site for atherosclerotic aneurysm formation.

**Clinical Pearl / High-Yield Fact**
Atherosclerotic aneurysms are more common in individuals with a history of smoking, hypertension, and hyperlipidemia. Early detection and management of these risk factors can help prevent the development of atherosclerotic aneurysms.

**Correct Answer: C. Abdominal aorta.**