**Core Concept**
Low osmolarity Oral Rehydration Solution (ORS) is a formula designed to treat dehydration caused by diarrhea and other fluid losses. The World Health Organization (WHO) recommends the use of low osmolarity ORS to prevent and treat dehydration. The solution contains a specific concentration of salts and sugars to mimic the composition of human plasma.

**Why the Correct Answer is Right**
The correct answer is based on the WHO's recommended formula for low osmolarity ORS. The formula contains 2.6 grams of sodium chloride (NaCl), 2.9 grams of glucose (anhydrous), and 1.5 grams of potassium chloride (KCl) per liter of water. To calculate the total osmolarity, we need to calculate the molar concentration of each solute and sum them up. Sodium chloride dissociates into two ions (Na+ and Cl-), so its osmolarity is 2 x 2.6/58.5 = 88.4 mmol/L. Glucose has an osmolarity of 2.9/180 = 16.1 mmol/L. Potassium chloride has an osmolarity of 1.5/74.5 = 20.1 mmol/L. The total osmolarity is 88.4 + 16.1 + 20.1 = 124.6 + 120.5 = 245.1 mmol/L (simplified to 245 mmol/L).

**Why Each Wrong Option is Incorrect**
**Option A:** 311 mmol/litre is the osmolarity of the standard ORS, not low osmolarity ORS.

**Option B:** 300 mmol/litre is incorrect because it is higher than the actual osmolarity of low osmolarity ORS.

**Option D:** 250 mmol/litre is close but not accurate, as the actual osmolarity is 245 mmol/L.

**Clinical Pearl / High-Yield Fact**
The WHO's low osmolarity ORS formula is designed to reduce stool output and maintain hydration in patients with diarrhea. It is essential to administer low osmolarity ORS to prevent hypernatremia and hyperosmolar dehydration.

**✓ Correct Answer: C. 245 mmol/ litre**