State one possible reason why molecule a could diffuse across the membrane but molecule b could not.

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.

passive transport is a movement of biochemicals and other atomic or molecular substances across cell membranes without need of energy input. unlike active transport, it does not require an input of cellular energy because it is instead driven by the tendency of the system to grow in entropy.

so, according to what that says, i believe it's hypertonic and hypotonic.

good luck!