The outer layer of a cell that acts as a gatekeeper to a cell is the plasma membrane.
The plasma membrane is the boundary between the cell and its environment. It regulates what enters and exits the cell. Cells must maintain an appropriate amount of molecules to function inside them.
The correct answer between all the choices given is the second choice or letter B. I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.