**Core Concept**
The reduced osmolarity Oral Rehydration Solution (ORS) was introduced to improve the efficacy of rehydration therapy, while minimizing the risk of hypernatremia and hyperchloremic metabolic acidosis. The new formula is designed to better mimic the composition of human sweat and intestinal fluid.

**Why the Correct Answer is Right**
The reduced osmolarity ORS has a sodium concentration of 75 mmol/L, which is lower than the original formula. This decrease in sodium concentration helps to reduce the risk of hypernatremia and hyperchloremic metabolic acidosis, making it a safer and more effective treatment for dehydration. The new formula is also designed to promote the absorption of sodium and water from the intestine, which is essential for rehydration.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is lower than the actual sodium concentration in reduced osmolarity ORS. Sodium levels that are too low can lead to hyponatremia, which can be just as dangerous as hypernatremia.
**Option B:** Incorrect because it is higher than the actual sodium concentration. A sodium concentration of 90 mmol/L would be more consistent with the original ORS formula.
**Option C:** Incorrect because it is significantly lower than the actual sodium concentration. A concentration of 30 mmol/L would be more consistent with a hypo-osmotic solution, which is not what is needed for rehydration.

**Clinical Pearl / High-Yield Fact**
The reduced osmolarity ORS is a significant improvement over the original formula, but it's essential to remember that ORS should be used in conjunction with other treatments, such as fluid restriction and electrolyte replacement, to manage dehydration effectively.

**Correct Answer: D. 75**