Osmosis is the movement of a solvent (usually water) across a semipermeable membrane from a region of high solvent concentration (low solute concentration) to a region of low solvent concentration (high solute concentration).

So, the solution that causes osmosis is the one with a higher solute concentration, or in other words, the more concentrated solution. This difference in concentration creates a concentration gradient that drives the movement of water molecules across the membrane.

Here's a breakdown:

* High solute concentration: This solution has a lower concentration of free water molecules because they are bound to the solute molecules.

* Low solute concentration: This solution has a higher concentration of free water molecules because fewer water molecules are bound to solute molecules.

The movement of water molecules from the high concentration to the low concentration aims to equalize the concentration of both solutions. This process is called osmotic pressure.

It's important to note that it's the difference in solute concentration between the two solutions that drives osmosis, not the absolute concentration of either solution.