Answer: A. Cell membrane
Explanation: Cell membrane is a biological lipid bilayer that separates the interior part of every cell from its exterior. The main components of cell membrane include lipids, proteins and carbohydrate moieties that are attached to either lipids or proteins.
Cytoskeleton contains series of intercellular proteins that gives shape and provides support to a cell. Enzymes are proteins that catalyze biological reactions. Receptors are proteins found mostly on the surface of a cell.
