Answer:
A. through proteins embedded in the membrane that "recognize" specific substrates and allow their passage.
Explanation:
Some of the membrane proteins serve as transport proteins. These transport proteins can serve as channels, carriers or pumps and allow facilitated diffusion or active transport of the substances across the membrane. ion channels are transport proteins that allow specific ions to move across the membrane since the charged ions cannot cross the non-polar core of the membranes. Most of the ions channels are specific and allow only particular ions to move through them. For example, Na+ ion channels will allow only Na+ ions to move across it while the chloride ions would not able to move through it.
