**Core Concept**
Conivaptan is a non-selective vasopressin receptor antagonist that plays a crucial role in the management of euvolemic and hypervolemic hyponatremia. It works by inhibiting the action of vasopressin (ADH) on its receptors in the kidneys, leading to increased water excretion and correction of serum sodium levels.

**Why the Correct Answer is Right**
Conivaptan selectively blocks vasopressin V1A and V2 receptors, preventing vasopressin-induced water reabsorption in the collecting ducts and vasopressin-induced vasoconstriction. This action results in an increase in free water clearance and a decrease in serum osmolality, making it an effective treatment for euvolemic and hypervolemic hyponatremia. Conivaptan is administered intravenously, and its use is associated with a significant increase in serum sodium levels.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct mechanism of action of conivaptan. While vasopressin receptor antagonists do play a role in hyponatremia management, the specific action of conivaptan is what sets it apart.

**Option B:** This option is incorrect because it implies that conivaptan is a selective V2 receptor antagonist, which is not accurate. Conivaptan is a non-selective antagonist that blocks both V1A and V2 receptors.

**Option C:** This option is incorrect because it suggests that conivaptan is used to treat hypovolemic hyponatremia. While conivaptan can be used to treat hyponatremia, its use is generally reserved for euvolemic and hypervolemic hyponatremia.

**Option D:** This option is incorrect because it implies that conivaptan is administered orally. Conivaptan is actually administered intravenously.

**Clinical Pearl / High-Yield Fact**
Conivaptan is a useful treatment option for euvolemic and hypervolemic hyponatremia, particularly in patients who are not responding to conventional therapy. It is essential to carefully monitor serum sodium levels when using conivaptan to avoid overcorrection and potential complications.

**Correct Answer:** C.