**Core Concept**
Hypotonic solutions cause red blood cells to swell and eventually burst due to an influx of water into the cells, leading to hemolysis. This occurs because the concentration of solutes in the hypotonic solution is lower than that inside the cells, resulting in an osmotic imbalance.

**Why the Correct Answer is Right**
The test solution is hypotonic because it causes the red blood cells to swell and burst. This indicates that the concentration of solutes in the test solution is lower than that inside the red blood cells. The osmotic pressure of the test solution is lower than that of the red blood cells, resulting in an influx of water into the cells. This is a classic example of osmosis, where water moves from an area of high concentration to an area of low concentration through a selectively permeable membrane.

**Why Each Wrong Option is Incorrect**
* **Option A:** Isotonic solutions have the same concentration of solutes as the red blood cells and do not cause swelling or bursting. This is the opposite of what is observed in this study.
* **Option B:** Hypertonic solutions have a higher concentration of solutes than the red blood cells, causing water to leave the cells and shrink. This is also the opposite of what is observed.
* **Option C:** Hypotonic solutions cause red blood cells to swell and burst, but this option does not specify the osmolarity of the test solution.

**Clinical Pearl / High-Yield Fact**
When assessing the tonicity of a solution, always consider the concentration of solutes relative to the cells. Hypotonic solutions can cause hemolysis, while hypertonic solutions can cause shrinkage.

**Correct Answer:** . Hypotonic.