Final answer:
The cell membrane's characteristic that regulates what enters and leaves the cell is selective permeability.
Step-by-step explanation:
The characteristic of a cell's membrane that allows it to regulate what enters and leaves the cell is known as selective permeability. This means that the membrane permits the passage of certain substances while restricting the passage of others. Selective permeability is achieved through various structures and processes, such as the presence of protein channels and transporters in the membrane. These channels and transporters help in allowing specific molecules to cross the membrane while excluding others.
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