Answer:
The outer layer of a cell that acts as a gatekeeper to a cell is the Cell Membrane or Plasma Membrane.
The cell membrane made up of mostly phospholipids and proteins and is known as selectively permeable membrane as it allows the entry of some selective molecules into it.
The plasma membrane is the layer present between the cell and its external environment. As the gatekeeper takes care of entry, cell membrane also regulates the molecules that enters and exits the cell from external environment.
Cells membrane helps in maintaining appropriate amount of molecules to function inside them.
Hence, the correct option is Cell Membrane.