**Core Concept**
The thoracic duct is the main lymphatic vessel responsible for draining lymph from the majority of the body into the bloodstream. It plays a crucial role in the immune system by transporting lymphocytes and other immune cells. The thoracic duct's course is an essential aspect of its function, particularly in relation to its crossing over certain anatomical structures.

**Why the Correct Answer is Right**
The thoracic duct crosses over the aorta at the aortic hiatus of the diaphragm, which is located at the level of the T12 vertebra. This crossing is a significant anatomical landmark that allows the thoracic duct to continue its course into the abdominal cavity. The thoracic duct's crossing over the aorta is facilitated by the aortic hiatus, a natural opening in the diaphragm that allows the aorta, thoracic duct, and other structures to pass through.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the thoracic duct does not cross over the vena cava at any point in its course. The vena cava is a separate and distinct vascular structure that does not have a direct relationship with the thoracic duct.

**Option B:** This option is incorrect because the thoracic duct does not cross over the subclavian artery at any point in its course. While the subclavian artery is a significant vascular structure in the neck, it is not directly related to the thoracic duct's crossing.

**Option D:** This option is incorrect because the thoracic duct does not cross over the inferior vena cava at any point in its course. The inferior vena cava is a separate and distinct vascular structure that does not have a direct relationship with the thoracic duct.

**Clinical Pearl / High-Yield Fact**
The thoracic duct's crossing over the aorta is an important anatomical landmark that can be remembered using the mnemonic "T12" to represent the level of the crossing. Additionally, the thoracic duct's course can be visualized using imaging studies such as CT or MRI, which can be useful in diagnosing lymphatic disorders.

**Correct Answer:** C.