**Core Concept**
The composition of Oral Rehydration Solution (ORS) is crucial for effective treatment of dehydration, particularly due to diarrhea. The World Health Organization (WHO) recommends a specific formula to ensure optimal absorption of fluids and electrolytes. Glucose plays a pivotal role in this process by facilitating the absorption of sodium and water in the small intestine.

**Why the Correct Answer is Right**
Glucose in ORS acts as an osmotic agent, which enhances the absorption of sodium and water through the sodium-glucose cotransporter 1 (SGLT1) in the small intestine. This cotransporter transports two sodium ions into the intestinal cells along with one glucose molecule. The glucose molecule is then absorbed into the bloodstream, while the sodium ions remain in the intestinal lumen. This movement of sodium ions into the intestinal cells creates an osmotic gradient, drawing water into the cells through a process called solvent drag. The absorbed water and electrolytes are then transported to the bloodstream, helping to rehydrate the patient.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct concentration of glucose in ORS, which is essential for optimal absorption.
**Option B:** This option is incorrect because it suggests the use of a different osmotic agent, which may not be as effective as glucose in facilitating sodium and water absorption.
**Option C:** This option is incorrect because it implies that glucose is not an essential component of ORS, which is contrary to the recommended formula.

**Clinical Pearl / High-Yield Fact**
The correct concentration of glucose in ORS is 13.5 grams per liter, which is essential for optimal absorption of sodium and water. This formula was developed by the WHO to ensure effective treatment of dehydration, particularly in children.

**Correct Answer: D. 13.5 grams per liter**