**Core Concept**
The World Health Organization (WHO) oral rehydration solution is a standardized formula designed to effectively treat dehydration caused by diarrhea and other fluid losses. It contains a balanced mixture of electrolytes and glucose, formulated to mimic the composition of human plasma.

**Why the Correct Answer is Right**
The correct answer is based on the WHO's recommended oral rehydration solution composition, which aims to replenish sodium and other electrolytes lost through diarrhea. The solution contains 75 mmol/L of sodium, which is crucial for maintaining proper water balance, nerve function, and muscle contraction. The sodium content helps to stimulate the absorption of glucose and water in the small intestine, thereby promoting rehydration.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the WHO oral rehydration solution contains more than 75 mmol/L of sodium, which would be excessive and potentially harmful.
* **Option B:** This option is incorrect because the recommended sodium content in the WHO oral rehydration solution is not 30 mmol/L, which would be insufficient to effectively replenish lost sodium.
* **Option C:** This option is incorrect because the sodium content in the WHO oral rehydration solution is not 150 mmol/L, which would be too high and potentially lead to hypernatremia.

**Clinical Pearl / High-Yield Fact**
The WHO oral rehydration solution is a lifesaving treatment for dehydration, but it should be used in combination with other measures, such as fluid restriction and antimicrobial therapy, to effectively manage diarrhea and its complications.

**Correct Answer:** C. 90