The rigth answer is cell membrane.
A cell membrane distinguishes a fundamental cellular structure formed from a double layer of phospholipids associated with other lipids, proteins and carbohydrates.
The membrane of the cell, also known as the plasma or cytoplasmic membrane, is a biological membrane (biomembrane) that separates the interior of all cells from the external environment.
The plasma membrane, cytoplasmic membrane or plasmalemma delimits the cytoplasm of the cell. Organelles are surrounded by a single cell membrane (peroxisomes, lysosomes, endoplasmic reticulum, dictyosomes, vacuole) or double (nucleus, plastids).