Osmotic pressure is the tendency for a single solution to absorb what substance?

The correct answer is B: Water. Osmotic pressure is the pressure required to prevent the flow of water across a semipermeable membrane to achieve equilibrium. Water moves from a region of lower solute concentration to higher solute concentration to balance the concentration on both sides of the membrane. Therefore, the tendency for a solution to absorb water is what osmotic pressure refers to. Choices A, C, and D are incorrect because osmotic pressure is specifically related to the movement of water molecules, not sodium, positive ions, or negative ions.