**Core Concept**
The underlying principle being tested is the movement of water across cell membranes due to **osmotic gradients**, which affects cell volume. This concept is crucial in understanding how red blood cells behave in different environments, particularly in relation to **osmolality** and **tonicity**.

**Why the Correct Answer is Right**
When a red cell is placed in a hypotonic solution (lower osmolality than the cell), water moves into the cell through **osmosis** to equalize the solute concentrations inside and outside the cell. Given the initial cell volume of 100 cu.microns and an intracellular osmolality of 300 mOsm, placing it in a 100 mOsm solution will cause water to rush into the cell, increasing its volume. The final volume can be calculated using the principle that the total number of osmoles remains constant, thus the product of the initial volume and osmolality equals the product of the final volume and the external osmolality.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice might assume no net movement of water, which is incorrect given the osmotic gradient.
**Option B:** This option underestimates the degree of swelling.
**Option D:** This choice incorrectly suggests the cell would shrink, which is what would happen in a hypertonic solution.

**Clinical Pearl / High-Yield Fact**
Remember, when a cell is placed in a hypotonic solution, it will swell due to water influx, and if the solution is isotonic, there will be no net change in cell volume. This concept is vital in understanding various clinical conditions and the management of fluid and electrolyte imbalances.

**Correct Answer:** C. 300 cu.microns