Once equilibrium is reached, .a. molecules move, but there is no net movement in a particular direction. b. passive transport starts over to create a concentration gradient. c. the membrane permits all molecules to freely move across the membrane. d. molecules no longer move.

molecules move, but there is no net movement in a particular direction.

Option A is correct.

Explanation:

When equilibrium is reached, particles of a solution will continue to move across the membrane in both directions. However, because almost equal numbers of particles move in each direction, there is no further change in concentration. Equilibrium is reached in a system when the concentration of a solute is equal on both sides of the membrane.