**Core Concept**
Normal saline, also known as 0.9% sodium chloride solution, is a commonly used isotonic solution for intravenous administration. The osmolarity of a solution is a measure of the concentration of osmotically active particles per unit volume, usually expressed in milliosmoles per liter (mOsm/L).

**Why the Correct Answer is Right**
The osmolarity of normal saline is calculated based on its sodium chloride concentration. Since normal saline contains 0.9 grams of sodium chloride per 100 mL, we can calculate its osmolarity as follows: 0.9 g/100 mL = 9 g/L. Sodium chloride dissociates into 2 osmotically active particles (sodium and chloride ions), so we multiply the concentration by 2 to get the total osmolarity: 9 g/L * 2 = 18 mOsm/L. However, normal saline is an isotonic solution, meaning it has the same osmolarity as human blood (approximately 300 mOsm/L). Therefore, the correct answer is not 18 mOsm/L, but rather a multiple of 0.9 g/100 mL.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is missing, so we cannot evaluate it.

**Option B:** This option is also missing, so we cannot evaluate it.

**Option C:** This option is missing, so we cannot evaluate it.

**Option D:** This option is also missing, so we cannot evaluate it.

**Clinical Pearl / High-Yield Fact**
Normal saline is an isotonic solution, which means it has the same osmolarity as human blood. This is why it can be administered intravenously without causing hemolysis or tissue damage.

**Correct Answer:** D. 308 mOsm/L (normal saline has the same osmolarity as human blood, approximately 308 mOsm/L).