Answer:
(a) Plasma Membrane:
The main function of the plasma membrane, also known as the cell membrane, is to serve as a selectively permeable barrier that separates the interior of a cell from its external environment. The plasma membrane regulates the passage of substances in and out of the cell, controlling the exchange of ions, nutrients, waste products, and other molecules. It plays a crucial role in maintaining the cell's internal environment, integrity, and homeostasis, while also facilitating communication with neighboring cells. Additionally, the plasma membrane contains various proteins and receptors that are involved in cell signaling, adhesion, and recognition. Overall, the plasma membrane is essential for the proper functioning and survival of a cell.