No matter what the cell type, prokaryotic or eukaryotic, plant or animal, the forms a protective barrier between the exterior environment and the interior of the cell.
a. cell wall
b. cytoskeleton
c. cell membrane
d. glycoproteins

c. cell membrane forms a protective barrier between the exterior environment and the interior of the cell.

Cell membranes contain membrane proteins that allows all the interaction that occurs within the cell. It makes the cell membrane selectively permeable because of its characteristics as being fluid or its fluidity which means to say that they can move freely within the lipid bilayer. The membrane proteins can also be stored in the lipid bilayer together with the enzymes. It cannot be denied that membrane proteins have a lot of functions based on its type. They can be integral proteins, peripheral proteins and lipid bound proteins.

When you are making a guess you are forming a hypothesis