**Core Concept**
The initial fluid of choice for diarrhea in infants is an isotonic solution that replaces lost electrolytes and fluids without causing further imbalances. This is crucial in preventing dehydration and maintaining proper fluid balance in infants, who have a high surface area-to-weight ratio and are prone to rapid fluid shifts.

**Why the Correct Answer is Right**
The correct answer is an isotonic solution, such as oral rehydration solution (ORS), which contains sodium, potassium, chloride, and glucose in a specific ratio. This solution helps to replace lost sodium and chloride ions, which are essential for maintaining proper fluid balance and preventing dehydration. The glucose in ORS also helps to promote sodium absorption in the small intestine, making it an effective treatment for diarrhea in infants.

**Why Each Wrong Option is Incorrect**
**Option A:** Breast milk is not a suitable initial fluid for treating diarrhea in infants, as it may not provide sufficient electrolytes and fluids to replace those lost due to diarrhea. While breast milk is an excellent source of nutrition, it may not be enough to prevent dehydration in infants with diarrhea.
**Option B:** 0.9% saline solution is not suitable for treating diarrhea in infants, as it contains too much sodium and may cause hypernatremia. Infants with diarrhea already have an increased sodium load, and administering more sodium can exacerbate the problem.
**Option C:** Water is not a suitable initial fluid for treating diarrhea in infants, as it may cause further dilution of electrolytes and lead to hyponatremia. Infants with diarrhea already have an increased risk of hyponatremia, and administering water can worsen this condition.

**Clinical Pearl / High-Yield Fact**
In infants with diarrhea, it's essential to use an isotonic solution that contains sodium, potassium, chloride, and glucose in a specific ratio. This helps to replace lost electrolytes and fluids, preventing dehydration and maintaining proper fluid balance.

**Correct Answer:** C. Oral Rehydration Solution (ORS)