**Core Concept**
The management of dehydration in children, particularly in the context of acute diarrhoea, involves the administration of oral rehydration therapy (ORT) to replace lost fluids and electrolytes. The World Health Organization (WHO) recommends a specific formula for calculating the amount of fluid required for rehydration.

**Why the Correct Answer is Right**
The formula for calculating the amount of fluid required for rehydration in children is based on the child's weight and the duration of rehydration. In this case, the child weighs 12kg and requires fluid replacement in the first 4 hours. The correct calculation involves using the formula: 100 mL/kg body weight for the first 4 hours. This formula is widely accepted and recommended by various medical organizations, including the WHO and the American Academy of Pediatrics (AAP).

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely incorrect because it does not take into account the child's weight or the duration of rehydration.
* **Option B:** This option may be incorrect because it does not provide a clear formula or calculation for determining the amount of fluid required.
* **Option D:** This option is likely incorrect because it does not correspond to the standard formula for calculating the amount of fluid required for rehydration in children.

**Clinical Pearl / High-Yield Fact**
When managing dehydration in children, it is essential to use the correct formula for calculating the amount of fluid required, as over- or under-replacement of fluids can lead to complications such as hypovolemic shock or electrolyte imbalances.

**Correct Answer:** C. 1200 mL