**Core Concept**
The diaphragm is the primary muscle involved in breathing, responsible for increasing the volume of the thoracic cavity and facilitating inhalation. As the diaphragm contracts, it flattens and descends, thereby increasing the volume of the thoracic cavity and reducing intrathoracic pressure.

**Why the Correct Answer is Right**
The correct answer is related to the anatomy and function of the diaphragm. The diaphragm is a dome-shaped muscle that separates the chest cavity from the abdominal cavity. When it contracts, it helps to decrease the pressure in the chest cavity, allowing air to flow into the lungs. The contraction of the diaphragm is also assisted by the contraction of the intercostal muscles between the ribs.

**Why Each Wrong Option is Incorrect**
* **Option A:** This statement is incorrect because the crura of the diaphragm are actually two tendinous structures that attach to the lumbar vertebrae and help to anchor the diaphragm in place.
* **Option B:** This statement is incorrect because the phrenic nerve is responsible for innervating the diaphragm, but it is not a direct action of the diaphragm itself.
* **Option C:** This statement is incorrect because the diaphragm is not responsible for increasing the pressure in the abdominal cavity; rather, it helps to decrease the pressure in the chest cavity.

**Clinical Pearl / High-Yield Fact**
The diaphragm is the most important muscle involved in breathing, and its function is essential for maintaining normal respiratory function. Damage to the diaphragm, such as that caused by a penetrating injury, can lead to respiratory failure and require mechanical ventilation.

**Correct Answer:** C.