## Core Concept
The question tests the understanding of osmolarity calculations for a given solution, specifically a 4.2% solution of sodium bicarbonate. Osmolarity is a measure of the concentration of osmotically active particles in a solution, expressed as the number of osmoles of solute particles per liter of solution. Sodium bicarbonate (NaHCO3) dissociates into two ions in solution: sodium (Na+) and bicarbonate (HCO3-).

## Why the Correct Answer is Right
To calculate the osmolarity of a 4.2% solution of sodium bicarbonate, we first need to know the molar mass of sodium bicarbonate and then calculate the molarity of the solution. The molar mass of NaHCO3 is approximately 84 g/mol. A 4.2% solution means there are 4.2 grams of NaHCO3 per 100 mL of solution.

First, convert the percentage solution to grams per liter: 4.2 g/100 mL * 10 = 42 g/L.

Then, calculate the molarity: Molarity = moles of solute / liters of solution = (42 g / 84 g/mol) = 0.5 M.

Since sodium bicarbonate dissociates into two particles (Na+ and HCO3-), the osmolarity is twice the molarity: 0.5 M * 2 = 1 osmole/L * 1000 = 1000 mosm/L.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not match our calculated osmolarity of 1000 mosm/L for a 4.2% sodium bicarbonate solution.
- **Option B:** This option is incorrect for the same reason as Option A; it does not align with the calculated osmolarity.
- **Option D:** This option suggests a much higher osmolarity than calculated, which does not align with the dissociation and concentration of a 4.2% sodium bicarbonate solution.

## Clinical Pearl / High-Yield Fact
A key point to remember is that when calculating osmolarity from molarity, you must consider the number of particles a solute dissociates into. For sodium bicarbonate, this results in an osmolarity that is twice its molarity. This concept is crucial in understanding the osmotic effects of different solutions in clinical settings.

**Correct Answer:** C. 1000.