The answer is A
To find the number of oxygen molecules on each side of the membrane after equilibrium, we need to understand the principles of diffusion and osmosis.
Osmosis is the spontaneous flow of a solvent through a semipermeable membrane from a solution of lower solute concentration towards a solution of higher solute concentration. In this case, the solute is an oxygen molecule.
At equilibrium, the solute will have diffused equally across the membrane until the net movement of the solute is zero. Therefore, the number of oxygen molecules on each side of the membrane will be the same.
Since oxygen is a gas molecule that can freely cross a membrane, it won't be affected by osmosis, and it will equilibrate on both sides of the membrane.
So the number of oxygen molecules on each side of the partition after equilibrium will be equal to each other. The answer is A) 3.