Which type of membrane allows certain molecules to pass based on their size?

The correct answer is selectively permeable. This type of membrane is designed to allow only certain molecules to pass through while restricting others. The selective permeability is primarily determined by the size, charge, and solubility of molecules.

For example, small, nonpolar molecules like oxygen can easily pass through the lipid bilayer of a selectively permeable membrane, while larger or polar molecules may not be able to cross without assistance from transport proteins. This feature is crucial for maintaining the right concentration of substances within the cell and facilitating the necessary exchanges with the environment.

Freely permeable membranes allow any substance to pass through without restriction, which is not the case here. Impermeable membranes prevent any substances from passing through, whereas partially permeable membranes may allow some molecules through but do not have the same specific selectivity as a selectively permeable membrane. This distinction is key to understanding how cells control their internal environment.