Answer : The reason is Osmosis.
Explanation :
Osmosis : It is a type of process in which the movement of particles occurs from its high concentration to low concentration through the selective semi-permeable membrane.
From the given image we conclude that the molecule A diffuses across the membrane but molecule B do not due to the osmosis. That means the selective semi-permeable membrane allows only molecule A through it.
Hence, the reason is Osmosis.