**Core Concept:**
Oral Rehydration Solution (ORS) is a solution used for rehydration therapy in cases of dehydration caused by diarrhea. It consists of balanced electrolytes and glucose to replace lost fluids and electrolytes in the body. The treatment of choice for mild to moderate dehydration due to diarrhea is ORS.

**Why the Correct Answer is Right:**
The correct answer, **D**, is based on calculating the fluid deficit needed to be replaced in the child. A 4 kg child with mild to moderate dehydration due to diarrhea would have lost around 100-150 ml/kg of fluid. Hence, for a 4 kg child, the fluid deficit would be 400-600 ml. An ORS bag contains about 400 ml of fluid, which is sufficient for the child's rehydration.

**Why Each Wrong Option is Incorrect:**
Option A is incorrect because it gives a fixed amount of 200 ml ORS, which may not be sufficient for the child's needs based on its weight and fluid loss.

Option B suggests giving 100 ml ORS every hour. While the administration frequency is correct, the volume is too low to effectively replace the fluid deficit in a child of this weight.

Option C provides a fixed amount of 400 ml ORS, which may be excessive for a child of this weight, potentially causing fluid overload.

**Why the Correct Answer is Right:**
**Correct Answer: D** is right because it calculates the sufficient amount of ORS needed for the child based on its weight and fluid loss. The given formula is:

Fluid deficit = (Body weight x fluid loss) / ORS bag volume

**Core Concept**:
In cases of dehydration due to diarrhea, ORS is a safe, effective, and cost-effective solution for rehydration therapy. ORS helps restore the body's electrolyte balance and replace lost fluids.

**Core Concept**:
In children with mild to moderate dehydration due to diarrhea, ORS is the first-line therapy. It is crucial to ensure the child receives an adequate amount of ORS to prevent complications such as fluid overload or electrolyte imbalances.