**Core Concept**
Osmolarity is a measure of the concentration of osmotically active particles in a solution. In the context of sodium bicarbonate, we need to calculate the osmolarity based on its molecular structure and ionic dissociation in solution.

**Why the Correct Answer is Right**
Sodium bicarbonate (NaHCO3) dissociates into sodium ions (Na+), bicarbonate ions (HCO3-), and a hydrogen ion (H+). The osmolarity of a solution is calculated by adding the contributions of each solute particle. Since sodium bicarbonate dissociates into three particles (Na+, HCO3-, and H+), its osmolarity is 3 times the molarity of the solute.

The molarity of a 4.2% solution of sodium bicarbonate can be calculated as follows:

Molarity = (4.2 g / 84 g/mol) x (1000 ml / 1 L) = 0.05 M

Since sodium bicarbonate dissociates into three particles, the osmolarity is 3 x 0.05 M = 0.15 Osm/L.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.

**Option B:** This option does not provide a valid calculation for osmolarity.

**Option C:** This option does not account for the dissociation of sodium bicarbonate into multiple particles.

**Option D:** This option does not provide a valid calculation for osmolarity.

**Clinical Pearl / High-Yield Fact**
When calculating osmolarity, remember to account for the dissociation of electrolytes into multiple particles, as this can significantly affect the overall osmolarity of the solution.

**Correct Answer: C. 0.15 Osm/L**