**Core Concept**
The diaphragmatic opening, also known as the aortic hiatus, is a passage through the diaphragm that allows the aorta, thoracic duct, and sympathetic trunks to travel between the thoracic and abdominal cavities. This anatomical structure is crucial for the passage of vital structures, enabling the continuity of circulation and lymphatic drainage between the thorax and abdomen.

**Why the Correct Answer is Right**
The correct answer is the structure that passes through the diaphragmatic opening. The aorta, being the major artery that arises from the left ventricle of the heart, descends through the thoracic cavity and passes through the diaphragmatic opening, entering the abdominal cavity. This passage is necessary for the aorta to continue its course, supplying oxygenated blood to the abdominal organs. The sympathetic trunks, responsible for transmitting sympathetic nerve impulses, also pass through the diaphragmatic opening, enabling the coordination of autonomic functions between the thorax and abdomen.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as there is no evidence that the esophagus passes through the diaphragmatic opening. The esophagus, a vital organ for swallowing, passes through the esophageal hiatus, a separate opening in the diaphragm.
* **Option B:** This option is incorrect as the vena cava, while an important structure, does not pass through the diaphragmatic opening. The vena cava, responsible for returning deoxygenated blood to the heart, has a separate opening in the diaphragm.
* **Option D:** This option is incorrect as the thoracic duct, while passing through the diaphragmatic opening, is not the primary structure associated with this passage.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that the aortic hiatus is located at the level of the T12-L1 vertebrae, making it an important landmark for surgical procedures and medical imaging studies.

**Correct Answer:** C.