Plasma membrane is fluid-structure due to the presence of
A. Carbohydrate
B. Lipid
C. Glycoprotein
D. Polysaccharide

Hint: Unsaturated fatty acids have the ability to push adjacent phospholipid molecules present in the cell membrane away, which helps to maintain fluidity in the membrane.

Complete answer:
The plasma membrane is fluid-structure due to the presence of lipid, cholesterol (mainly), and it’s mosaic nature.

Additional Information
1. The plasma membrane or the cell membrane consists of a lipid bilayer that is semipermeable. It regulates the transport of materials entering and exiting the cell.
2. The integral proteins and lipids exist in the membrane as separate but loosely attached molecules along with glycoprotein, sugar, sialic acid, etc.
3. Unsaturated fatty acids consist of multiple bonds, and lesser hydrogen atoms attached, which results in fluidity.
4. Saturated fatty acids have no double bonds in the hydrocarbon chain and the maximum amount of hydrogen. The absence of double bonds decreases fluidity.
5. ‘Cholesterol’ functions as a buffer, holding the phospholipids molecules together and preventing lower temperatures from inhibiting fluidity and preventing higher temperatures from increasing fluidity. Thus, cholesterol is the most important factor responsible for the regulation of fluidity of the plasma membrane.
6. The fluidity is also affected by temperature. When it’s cold they are found closer together and when it’s hot they move farther apart.

Fluidity is important for many reasons:
1. It allows membrane proteins to move rapidly in the lipid bilayer.
2. It permits membrane lipids and proteins to move in and out of bilayer.
3. It enables membranes to fuse with one another.

So, the correct answer is ‘lipid'.

Note:
1. Glycoproteins also provide channels for the entry of molecules but they do not provide fluidity.
2. Each double bond present in unsaturated fatty acids provides a bend of 30° to the molecule which makes it flexible to move.