**Core Concept**
The standard World Health Organization (WHO) oral rehydration solution is a critical tool for treating dehydration caused by diarrhea, particularly in children. The solution is designed to mimic the electrolyte composition of human plasma and intestinal fluid, aiming to balance sodium levels and prevent excessive loss.

**Why the Correct Answer is Right**
The correct concentration of sodium ions in the WHO oral rehydration solution is 75 millimoles per liter (mmol/L). This concentration is based on the concept of "osmotic balance," where the solution's sodium content helps to maintain the balance of fluids and electrolytes in the body. The sodium ions in the solution help to replenish the lost sodium ions in the body, thereby preventing excessive loss of water and electrolytes. The WHO formula takes into account the sodium content of human milk, which has a concentration of approximately 60-70 mmol/L.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too low, which can lead to sodium depletion and worsening dehydration.
**Option B:** This option is incorrect because it is too high, which can lead to hypernatremia and potentially cause seizures or other complications.

**Clinical Pearl / High-Yield Fact**
The WHO oral rehydration solution is a critical tool for treating dehydration, and its composition is carefully balanced to mimic the electrolyte composition of human plasma and intestinal fluid. The solution's effectiveness relies on the correct concentration of sodium ions, which is essential for maintaining osmotic balance.

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