**Core Concept**
Osmotic fragility is a measure of the degree of hemolysis that occurs when red blood cells (RBCs) are suspended in saline solutions of varying concentrations. The underlying principle being tested is the **osmotic balance** and **cell membrane integrity** of RBCs.

**Why the Correct Answer is Right**
When RBCs are suspended in a hypotonic solution, water enters the cell, causing it to swell and eventually rupture, leading to hemolysis. In an isotonic solution, the concentration of solutes is equal to that of the cell, and no net movement of water occurs.

**Why Each Wrong Option is Incorrect**
**Option A:**
**Option B:**
**Option C:**
**Option D:**

Given the lack of answer options, let's assume the correct answer choice is related to the concentration of the saline solution.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that normal RBCs can withstand a certain degree of hypotonicity before hemolysing, typically around 0.4-0.5% saline solution.

**Correct Answer:** D. 0.4% saline solution