I'm fairly certain your answer is: C. (The cell membrane <u>controls what enters and leaves the cell.</u>)
Explanation:
In the definition of a cell membrane it says that its function is to protect the integrity of the interior of the cell by <u>allowing certain substances into the cell while keeping other substances out.</u> With this definition in mind, C would make the most sense.
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.
