Explanation:

When a cell is placed in a solution, the movement of water molecules takes place due to a difference in concentration between the cell and the solution. This movement of water molecules causes changes in the size of the cell.

Hypotonic Solution:
A hypotonic solution is a solution with a lower concentration of solutes than the cell. When a cell is placed in a hypotonic solution, water molecules move from the solution into the cell causing the cell to swell up. This happens because the concentration of solutes inside the cell is higher than the concentration of solutes in the solution.

Isotonic Solution:
An isotonic solution is a solution with the same concentration of solutes as the cell. When a cell is placed in an isotonic solution, there is no net movement of water molecules. The cell remains the same size.

Hypertonic Solution:
A hypertonic solution is a solution with a higher concentration of solutes than the cell. When a cell is placed in a hypertonic solution, water molecules move from the cell into the solution causing the cell to shrink. This happens because the concentration of solutes inside the cell is lower than the concentration of solutes in the solution.

In this case, the cell swells up, which means that the water molecules are moving into the cell. Since the cell is swelling up, the solution must have a lower concentration of solutes than the cell. Therefore, the solution is hypotonic.

Conclusion:
Hence, the correct answer is option C, hypotonic.