**Core Concept**
The hypothalamus plays a crucial role in regulating body fluids, temperature, and hunger. The hypothalamic region responsible for thirst regulation is the **hypothalamic thirst center**, which responds to changes in osmolality by stimulating thirst and vasopressin release.

**Why the Correct Answer is Right**
The injection of hypertonic saline into the hypothalamic thirst center activates osmoreceptors, which detect changes in blood osmolality. This activation triggers a neural signal that stimulates the release of vasopressin (ADH) and the sensation of thirst. The **supraoptic nucleus** and **paraventricular nucleus** are key regions involved in this process.

**Why Each Wrong Option is Incorrect**
**Option A:** Injection of hypertonic saline into the **lateral hypothalamus** would stimulate appetite, not thirst. The lateral hypothalamus is involved in regulating hunger and food intake.
**Option B:** The **perifornical area** is involved in regulating sleep and arousal, not thirst.
**Option C:** The **ventromedial hypothalamus** is involved in regulating appetite and body weight, not thirst.

**Clinical Pearl / High-Yield Fact**
The hypothalamic thirst center is sensitive to changes in blood osmolality, and its stimulation can lead to intense thirst and vasopressin release. This is why patients with **diabetes insipidus** (due to vasopressin deficiency) often experience polydipsia and polyuria.

**Correct Answer:** C.