Mendel was studying how genes are past on to future generations and in what circumstances this can occur. He used pea plants as they multiply and grow quickly allowing him to observe many generations much faster than he would be able to observing humans or even other mammals. He also used pea plants because this way he could control reproduction of his pea plant population to adjust to specific questions he had in terms of his research.
My guess would be H₂O/ water, since osmosis is a form of passive transport and I can't see the model the question references. Gases like O₂/ oxygen, CO₂/ carbon dioxide, and simple sugars like glucose can also pass through a cell membrane with passive transport though.
