Answer: The correct answer for the fill in the blank is Cell membrane.
Cell membrane ( also called plasma membrane) is the membrane around cytoplasm of the cell. It is present in the cells of all living organisms. It forms a protective barrier of the cell as it separates interior components from the external environment of the cell.
It is a selectively permeable membrane as it controls the transport of substances in and out of the cell. By doing this, it does not allow the harmful substances to enter the cell.