**Core Concept**
Antidiuretic hormone (ADH), also known as vasopressin, is released by the posterior pituitary in response to increased plasma osmolarity and/or decreased intravascular volume. Its primary role is to promote water reabsorption in the collecting ducts of the kidneys, thereby concentrating urine and maintaining fluid balance.

**Why the Correct Answer is Right**
Hypovolemia, or low blood volume, triggers the renin-angiotensin-aldosterone system (RAAS), which leads to vasoconstriction and increased blood pressure. This drop in intravascular volume is detected by baroreceptors in the carotid and aortic sinuses, which signal the hypothalamus to release ADH. ADH enhances water reabsorption in the kidneys to restore blood volume and pressure. This mechanism is critical in maintaining homeostasis during dehydration or hemorrhage.

**Why Each Wrong Option is Incorrect**
Option A: Hypervolemia leads to decreased ADH secretion because increased blood volume suppresses the need for water conservation.
Option B: Hypertension alone does not directly stimulate ADH; it is hypovolemia that drives ADH release via volume depletion.
Option D: Decreased osmolarity reduces ADH secretion because ADH is released only when plasma osmolarity rises above 290 mOsm/kg.

**Clinical Pearl / High-Yield Fact**
ADH is released in response to **both** hypovolemia and hyperosmolarity — but hypovolemia is a key physiological trigger, especially in shock or dehydration. Remember: "Low volume → ADH ↑" is a classic exam trap — students often confuse it with osmolarity.

✓ Correct Answer: C. Hypovolemia