**Core Concept**
The aortic hiatus is an anatomical opening in the diaphragm through which several vital structures pass from the thoracic cavity to the abdominal cavity. This hiatus is a crucial passage for several nerves, veins, and lymphatic vessels.

**Why the Correct Answer is Right**
The correct answer involves identifying the structures that pass through the aortic hiatus. The aortic hiatus is located at the level of the T12 vertebra, and it allows the aorta, thoracic duct, azygos vein, and hemiazygos vein to pass from the thoracic cavity to the abdominal cavity. The sympathetic trunk, on the other hand, passes through the crura of the diaphragm, not the aortic hiatus.

**Why Each Wrong Option is Incorrect**
**Option A:** The aorta does pass through the aortic hiatus, but it's not the only correct answer, so this is a distractor.
**Option B:** The azygos vein does indeed pass through the aortic hiatus.
**Option C:** The hemiazygos vein also passes through the aortic hiatus.
**Option D:** The thoracic duct passes through the aortic hiatus.

**Clinical Pearl / High-Yield Fact**
The aortic hiatus is an important anatomical landmark in thoracic and abdominal surgery, as several vital structures pass through this opening.

**Correct Answer:** C. Hemiazygos vein