**Core Concept**
Hypernatremic dehydration occurs when the body loses water and sodium is retained, leading to an elevated serum sodium concentration. The initial goal of treatment is to correct the hypernatremia by administering fluids that are hypotonic to the serum.

**Why the Correct Answer is Right**
The correct answer is **Half Normal Saline (0.45% saline)**. This is because half normal saline is isotonic to the serum in terms of sodium, but has a lower osmolality than the serum, which helps to dilute the serum sodium concentration. Administering half normal saline helps to correct the hypernatremia without causing a rapid shift of water into the cells, which can lead to cerebral edema.

**Why Each Wrong Option is Incorrect**
**Option A:** **Normal Saline (0.9% saline)** is incorrect because it has the same sodium concentration as the serum, and administering it would not help to dilute the serum sodium concentration.
**Option B:** **D5W (5% dextrose in water)** is incorrect because it has a higher osmolality than the serum, which can exacerbate the hypernatremia by causing a shift of water out of the cells.

**Clinical Pearl / High-Yield Fact**
In the treatment of hypernatremic dehydration, it's essential to remember that the rate of correction should not exceed 1-2 mEq/L per hour to avoid cerebral edema.

**Correct Answer:** **C. Half Normal Saline (0.45% saline)**