## Core Concept
The osmolarity of a solution is a measure of the concentration of osmotically active particles in a solution, expressed as the number of osmoles of solute particles per liter of solution. The standard Oral Rehydration Solution (ORS) is used to treat dehydration caused by diarrhea. Its osmolarity is crucial for effective rehydration without causing an osmotic imbalance.

## Why the Correct Answer is Right
The correct osmolarity of the standard ORS solution, as recommended by the World Health Organization (WHO), is 245 mOsm/L. This osmolarity was reduced from the original formula to make it more effective and safer for patients, reducing the sodium and glucose concentrations. The reduced osmolarity ORS has been shown to decrease the stool output, reduce the need for intravenous fluids, and decrease the risk of hyponatremia.

## Why Each Wrong Option is Incorrect
- **Option A:** 100 mOsm/L - This osmolarity is too low for an effective ORS. Solutions with such low osmolarity would not efficiently promote the absorption of sodium and water in the intestines.
- **Option B:** 300 mOsm/L - While closer, this is still higher than the recommended osmolarity for the standard ORS. A solution with this osmolarity could potentially cause an osmotic imbalance.
- **Option D:** 500 mOsm/L - This osmolarity is significantly higher than what is recommended for ORS. Such a high osmolarity could exacerbate dehydration and electrolyte imbalances.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the reduced osmolarity ORS (245 mOsm/L) is the current standard recommended by the WHO and UNICEF. This change in osmolarity has been a significant update in the management of dehydration due to diarrhea. Remembering that the standard ORS osmolarity is around 245 mOsm/L can help prevent errors in clinical practice and ensure effective treatment of dehydration.

**Correct Answer: C. 245 mOsm/L.**