Answer:
D. proteins
Step-by-step explanation:
The plasma membrane is formed mainly by lipids and proteins. Lipids act by guaranteeing the membrane structure, while proteins are related to the main functions performed by this cell structure.
The proteins present in the plasma membrane are embedded in the bilayer. These proteins may be inserted totally or only partially into the membrane. It is worth mentioning that some act as true channels for the passage of substances. That is, we can say that membrane proteins are the molecules responsible for helping to control the movement of substances across cell membranes.