A. Ion pumps.
Selective permeability refers to the ability of the cell membrane to allow certain molecules or ions to pass through while restricting others. This is accomplished through various mechanisms, including passive transport (such as osmosis) and active transport (such as ion pumps). Ion pumps are a type of active transport mechanism that use energy to move ions across the cell membrane against their concentration gradient. This allows the cell to maintain a specific ion concentration inside the cell that is different from the concentration outside the cell.