Final Answer:
Conclusion about the solution is that the solution is B. isotonic.
Step-by-step explanation:
When a cell placed in a solution maintains its size, it indicates that the solution is isotonic. Isotonic solutions have the same concentration of solutes as the cell's cytoplasm, resulting in no net movement of water across the cell membrane.
In an isotonic solution, the osmotic pressure inside and outside the cell is balanced. This equilibrium prevents any significant change in the cell's volume. If the solution were diluted (option A), the cell would likely swell as water moves into the cell to equalize the concentration. Conversely, if the solution were hypotonic (option C), the cell would shrink due to water moving out of the cell. Similarly, a hypertonic solution (option D) would cause the cell to shrink as water moves from the cell into the more concentrated solution.
Understanding the concept of tonicity is crucial in biology, especially in contexts like cell biology and physiology. Isotonic solutions are essential in maintaining cell shape and function by ensuring a stable environment where water movement does not disturb cellular integrity.
In conclusion, the correct answer is B. The solution is isotonic, as evidenced by the cell maintaining its size when placed in the unknown solution.