**Core Concept**
Sodium concentration in oral rehydration solutions (ORS) plays a crucial role in maintaining proper fluid balance and correcting electrolyte imbalances in patients with diarrhea. The World Health Organization (WHO) has developed guidelines for ORS formulation to optimize rehydration therapy.

**Why the Correct Answer is Right**
The reduced osmolarity ORS was introduced in 2002 to reduce the risk of hypernatremia and hyperchloremia associated with the original ORS. The correct concentration of sodium in reduced osmolarity ORS is 75 mmol/L. This lower concentration helps to reduce stool output and prevent hypokalemia. The reduced osmolarity ORS also contains 75 mmol/L of sodium and 34 mmol/L of potassium per liter, which helps to maintain a proper balance of electrolytes.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the sodium concentration in reduced osmolarity ORS is not 90 mmol/L, which is higher than the recommended concentration.
* **Option B:** This option is incorrect because the sodium concentration in reduced osmolarity ORS is not 60 mmol/L, which is lower than the recommended concentration.
* **Option C:** This option is incorrect because the sodium concentration in reduced osmolarity ORS is not 120 mmol/L, which is higher than the recommended concentration.

**Clinical Pearl / High-Yield Fact**
Remember that the reduced osmolarity ORS was introduced to reduce the risk of hypernatremia and hyperchloremia associated with the original ORS. This formulation helps to maintain proper fluid balance and prevent complications in patients with diarrhea.

**Correct Answer: C. 75 mmol/L.**