**Core Concept**
The management of diarrhea in children involves rehydration and electrolyte replacement. The World Health Organization (WHO) recommends the use of Oral Rehydration Solution (ORS) to treat dehydration caused by diarrhea. Plan A is a specific protocol for the treatment of mild to moderate dehydration.

**Why the Correct Answer is Right**
According to the WHO protocol, Plan A recommends administering 75 mEq/L of sodium in the ORS. For children aged 0-2 years, the recommended amount of ORS to be given after each stool is 50-100 mL. However, since the child in the question is 5 years old, the recommended amount is 100 mL, as the child has reached the upper limit of the age group. This is based on the assumption that a child aged 5 years can tolerate a larger volume of ORS. The use of ORS helps to replace lost electrolytes and fluids, thereby preventing dehydration.

**Why Each Wrong Option is Incorrect**
**Option A:** 50 mL is below the recommended amount for a child aged 5 years, who falls into the upper age group.
**Option C:** 200 mL is excessive and may lead to over-rehydration, which can be life-threatening.
**Option D:** Giving ORS according to thirst is not a recommended approach, as it may lead to inadequate or excessive fluid intake.

**Clinical Pearl / High-Yield Fact**
When treating diarrhea in children, it's essential to use the correct ORS protocol and volume to prevent dehydration and electrolyte imbalance. Always refer to the WHO guidelines for the most up-to-date recommendations.

**✓ Correct Answer: B. 100 ml**