Various characteristics of the lipids in a membrane contribute to its fluidity-- which in turn determines how easily molecules can diffuse into and out of the cell across the membrane. Which of the following statements about this topic are correct?

A. Longer fatty acid tails of phospholipids make the membrane less fluid, thus slowing diffusion.

B. A higher concentration of saturated fatty acid tails of phospholipids makes the membrane less fluid, thus slowing diffusion.

C. At ambient temperatures (temperatures typically found in a cell), cholesterol stiffens the membrane, reducing fluidity and diffusion.

D. A higher concentration of unsaturated fatty acid tails of phospholipids makes the membrane less fluid, thus slowing diffusion.

Lipid bilayer is made of two layers of phospholipids, where each phospholipid molecule is made up of a glycerol which is attached to a phosphate group and two fatty acid chains by ester bonds.
The phosphate groups being hydrophilic are directed towards the outside of the bilayer. The fatty acid chains (saturated or unsaturated) being hydrophobic are directed towards the inside of the bilayer.
These fatty acid chains are not static and are capable of moving laterally in the bilayer, giving the bilayer the characteristics of fluidity.
The fluidity is mainly provided by the presence of unsaturated (with one or multiple double bonds) fatty acid. The presence of double bonds results in the fatty acid molecules to have a bent shape and this increases the area occupied by each such unsaturated fatty acid molecule. The free area available allows the lateral movement of the fatty acid molecules.
According to option A, long chain fatty acids tend to occupy more space within the lipid bilayer. This reduces the free space available for the movement of the lipid molecules.
According to option B, saturated fatty acids are those which lacks any double bonds and the carbon atoms are attached by single bonds. Therefore, these molecules are straight in shape and occupy maximum area in the lipid bilayer. This reduces the space available for the free movement of fatty acid molecules, thereby making the membrane less fluid in nature.
According to option C, cholesterol is a type of lipid molecule. They remain interspersed in between lipid molecules. At high temperatures, they prevent the lipid molecules from getting excessively fluid by filling in the gaps in between the lipid molecules and at very low temperatures they prevent the stiffening of the lipid molecules by getting associated with each other. At room temperature, they stabilise the lipid molecules between whom they remain interspersed and also reduce the free space in between them thereby reducing the lateral movement. This way they reduce the fluidity.