**Anatomical Passage**
The diaphragm is a dome-shaped muscle that separates the chest cavity from the abdominal cavity. At various levels, different structures pass through the diaphragm to connect the two cavities. Understanding the anatomical relationships of these structures is crucial for medical professionals.

**Why the Correct Answer is Right**
The thoracic duct is a vital lymphatic vessel that collects lymph from the lower body and returns it to the venous system. It passes through the diaphragm at the level of T12 (thoracic vertebra 12) to connect the abdominal and thoracic cavities. This passage allows the thoracic duct to drain lymph from the lower body into the bloodstream. The thoracic duct is a crucial structure in the lymphatic system, and its passage through the diaphragm is a key anatomical feature.

**Why Each Wrong Option is Incorrect**
**Option A:** The esophagus passes through the diaphragm at the esophageal hiatus, which is located at the level of T10. This is lower than the level of the thoracic duct.

**Option B:** The inferior vena cava (IVC) passes through the diaphragm at the level of T8, not T12. This is higher than the level of the thoracic duct.

**Option D:** The phrenic nerve passes through the diaphragm at various points, but it does not pass through the diaphragm at the level of T12. The phrenic nerve is primarily responsible for innervating the diaphragm and is not a structure that passes through the diaphragm as a vessel.

**Clinical Pearl / High-Yield Fact**
The thoracic duct is a key structure in the lymphatic system, and its passage through the diaphragm is an important anatomical feature. Understanding the level at which the thoracic duct passes through the diaphragm is crucial for medical professionals, particularly surgeons and radiologists.

**✓ Correct Answer: C. Thoracic duct**