<span>The answer is D - unlimited flow of water into or out of the cell. The plasma membrane is selectively permeable, meaning that it is tasked with controlling what substances cross it. It not only controls the transport and flow of water, but of other importance substances as well; this regulatory action is one of the principle functions of the membrane.</span>
Plasma membrane is a part of the cell made of a lipid bilayer and an embedded protein layer. It protects the cell from the surroundings and also controls the materials leaving or entering the cell. The membrane is semipermeable and therefore allows certain materials to enter or leave the cell. In this case, the plasma membrane may prevent unlimited flow of water into or out of the cell.
Cell transport is useful because it's, essentially, the movement of cells across the blood/veins. Active transport uses energy, while passive transport does not.
