This system is composed of two sections connected by a bridge and we have two molecules on this side and eight molecules on this side. What entropy tells us is that eventually three of these molecules will end up on this side and that this system will be the most probable system. So we want a system that's even not uneven or balanced, not on balance, in which we have five molecules here and five molecules here. Let's look at system C.
Now, in system C we basically have a cell. And this cell is surrounded by semipermeable membrane that allows water molecules in and out but does not allow any size molecules to go in or out. Now, in this cell, however, we have a small hole in the membrane and this hole is large enough to allow the molecules in and out.