Correct answer: B). Receptor sites
The cell membrane is made up of phospholipid and cholesterol. The main function of cholesterol molecule is to provide extra support to the cell, it is more hard than other lipids in the membrane. It helps to immobilize some lipid molecules around them. These substances bind on the receptor sides of the cell membrane and help in the transfer of molecule across the membrane.
Hence, the correct answer would be option B.