**Core Concept**
The World Health Organization (WHO) oral rehydration solution is a standard treatment for dehydration caused by diarrhea. It is designed to provide essential salts and electrolytes to replenish lost fluids. The sodium content in the WHO oral rehydration solution is crucial for maintaining proper fluid balance and preventing complications such as hyponatremia or hypernatremia.

**Why the Correct Answer is Right**
The WHO oral rehydration solution contains sodium chloride (NaCl) at a concentration of 2.6 grams per liter of solution. This amount of sodium is essential for maintaining proper fluid balance and facilitating the absorption of glucose in the small intestine. The sodium ions also help in regulating the amount of water in the body by influencing the osmotic balance across cell membranes. The correct sodium content in the WHO oral rehydration solution is 75 mmol/L.

**Why Each Wrong Option is Incorrect**
**Option A:** 30 mmol/L - This concentration is too low to effectively replenish lost sodium and maintain proper fluid balance.
**Option B:** 120 mmol/L - This concentration is too high and can lead to hypernatremia, a condition characterized by elevated sodium levels in the blood.
**Option C:** 90 mmol/L - Although closer to the correct answer, this concentration is still slightly higher than the recommended 75 mmol/L.

**Clinical Pearl / High-Yield Fact**
Remember the "WHO" formula for oral rehydration solution: 75 mmol/L sodium, 75 mmol/L potassium, and 2.6 grams of NaCl per liter of solution. This formula is essential for treating dehydration caused by diarrhea and preventing complications.

**Correct Answer:** B. 75