**Core Concept**
The diaphragm is a dome-shaped muscle that separates the chest cavity from the abdominal cavity. It has several openings, known as foramina, which allow various structures to pass through from the thoracic cavity to the abdominal cavity. The opening in the diaphragm opposite to the T10 vertebrae is known as the aortic hiatus.

**Why the Correct Answer is Right**
The aortic hiatus is a significant structure in the diaphragm, allowing the aorta, the thoracic duct, and the sympathetic trunk to pass from the thoracic cavity to the abdominal cavity. The aorta, in particular, passes through this hiatus to continue its course as the abdominal aorta. This is a critical anatomical feature, as the abdominal aorta supplies blood to the abdominal organs and the lower limbs.

**Why Each Wrong Option is Incorrect**
* **Option A:** The esophageal hiatus, located opposite to the T10 vertebrae, is incorrect because it primarily allows the esophagus and the vagus nerve to pass through.
* **Option B:** The caval opening, also known as the vena caval opening, is incorrect because it is located opposite to the T8 vertebrae and allows the inferior vena cava to pass through.
* **Option D:** The crura of the diaphragm, which are two thickened tendinous structures, are incorrect because they do not pass through the diaphragm's opening.

**Clinical Pearl / High-Yield Fact**
The aortic hiatus is an important landmark for surgeons and radiologists, as it can be used to identify the location of the abdominal aorta and surrounding structures during imaging studies or surgical procedures.

**Correct Answer: C. The aortic hiatus.**