Stimulation of the membrane causes it to become highly permeable to sodium ions, which diffuse rapidly to the inside due to the concentration gradient. The inside of the membrane becomes positively charged and the outside negatively charged. The membrane is then said to be depolarized. This constitutes the action potential. The reverse of the polarity of the membrane causes the beginning of repolarization.
The correct answer is D. Cell Membrane because the cell membrane contains two phospholipid layers with the protein molecules attached to them throughout.