What is osmotic pressure?

Osmotic pressure is the pressure required to stop the flow of water across a semipermeable membrane, which separates solutions of different concentrations. In this context, the passing of water from an area of low-salt concentration to an area of high concentration harmonizes with the concept of osmosis, whereby water moves to balance solute concentrations on either side of a membrane. This movement occurs naturally as water seeks to dilute the higher concentration of salt, illustrating how osmotic pressure is essential in various biological and chemical processes.

The other options refer to concepts that do not accurately represent osmotic pressure. For instance, the involvement of air moving between salt concentrations is not pertinent, as osmotic pressure strictly relates to the movement of water. Additionally, the comparisons drawn with atmospheric pressure and the mention of ozone are irrelevant to the definition of osmotic pressure, which is focused purely on solutions and the behavior of water in response to solute concentrations. Thus, the answer clearly capturing the essence of osmotic pressure is the passing of water from an area of low-salt concentration to an area of high concentration.