Final answer:
Cells maintain their size in a dynamic equilibrium when placed in an isotonic solution where the osmotic pressure is balanced.
Step-by-step explanation:
Growth, reproduction, and dynamic homeostasis require cells to create and maintain internal environments that differ from their external environments. The external conditions that would cause a cell to maintain its size in a dynamic equilibrium are those in which the osmotic pressure of the fluid inside the cell is equal to the osmotic pressure of the external solution.
This can be achieved when the cell is placed in an isotonic solution, where the flow of water into and out of the cell is balanced. As a result, the cell retains its size because there is no net movement of water.