Sodium and chloride ions are concentrated in the extracellular fluid (ECF), while potassium ions are concentrated inside cells.

A. There is an excessive amount of free water on one side: A high concentration of free water on one side creates a significant osmotic gradient. This would lead to rapid and substantial water movement toward the side with higher solute concentration. Minimal movement implies the absence of such a gradient.

B. The solutes have all converted to free water: Solutes are distinct chemical entities and do not spontaneously transform into solvent molecules. This explanation lacks a scientific basis in chemistry or physics. Osmosis depends entirely on the presence of stable, non-penetrating solutes to drive movement.

C. The membrane has too many aquaporins: A high density of aquaporins would increase the permeability of the membrane to water. If a concentration gradient existed, this would result in faster, not minimal, water movement. Minimal movement suggests a lack of driving force, not a lack of channels.

D. Both sides of the membrane have an equal solute concentration (isotonic), resulting in no net movement of water: When the osmolarity is identical on both sides, the chemical potential of water is balanced. While individual molecules still cross the membrane, the net flux is zero. This equilibrium state prevents observable changes in fluid volume.

E. The solution on both sides of the membrane is hypertonic: The term hypertonic is relative and requires a comparison between two different environments. If both sides have the same high concentration, they are isotonic to each other. The absolute concentration does not matter as much as the gradient.

True: The Na+/K+-ATPase pump actively maintains high extracellular sodium and high intracellular potassium concentrations. This electrochemical gradient is essential for secondary active transport and the resting membrane potential. Chloride typically follows sodium to maintain overall electrical neutrality.