What type of function does the cell membrane perform with regards to substances?

The cell membrane performs the function of regulating and controlling the entry and exit of substances, which is essential for maintaining homeostasis within the cell. This selective permeability allows the cell to manage its internal environment, ensuring that essential nutrients can enter, waste products can be expelled, and harmful substances are kept out. This function is facilitated by various mechanisms such as passive transport, active transport, and endocytosis/exocytosis, depending on the concentration gradients and the nature of the substances involved.

The other choices do not accurately describe the role of the cell membrane. While some substances can pass through the membrane freely, it is not a blanket allowance for all substances. Nor is it correct to say that the cell membrane does not allow any substances to pass, as this would imply complete impermeability, contradicting the membrane's function. Lastly, the cell membrane does not exclusively facilitate the movement of gases; it regulates a variety of substances, including ions and larger molecules, depending on their characteristics. Therefore, the correct choice clearly reflects the dynamic and regulatory nature of the cell membrane with respect to substances.