**Core Concept**
The fluidity of the plasma membrane is influenced by the type and ratio of its lipid components. The fluidity is determined by the ability of the lipid molecules to move laterally within the membrane, with more fluid membranes having higher lateral mobility.

**Why the Correct Answer is Right**
Arachidonic acid is an unsaturated fatty acid, which means it has one or more double bonds in its hydrophobic tail. The presence of double bonds in arachidonic acid leads to a kink in the hydrophobic tail, preventing the molecules from packing tightly together. This results in increased fluidity of the plasma membrane, as the arachidonic acid molecules can move more freely within the membrane. This effect is due to the reduced van der Waals forces between the arachidonic acid molecules, allowing them to maintain a higher degree of lateral mobility.

**Why Each Wrong Option is Incorrect**
**Option B:** Cholesterol is actually a fluidizer of the plasma membrane in low concentrations, but as its concentration increases, it can act as a rigidizer by packing between the phospholipid molecules and reducing the fluidity of the membrane.
**Option C:** Palmitic acid is a saturated fatty acid with no double bonds in its hydrophobic tail. This results in a straight, rod-like shape that allows the molecules to pack tightly together, reducing the fluidity of the plasma membrane.
**Option D:** Stearic acid is also a saturated fatty acid, similar to palmitic acid, with a straight, rod-like shape that promotes tight packing and reduced fluidity of the plasma membrane.

**Clinical Pearl / High-Yield Fact**
In addition to influencing membrane fluidity, the type of fatty acid present in the plasma membrane can also affect the activity of membrane-bound enzymes and receptors, highlighting the importance of maintaining the correct balance of fatty acid composition in cellular membranes.

**✓ Correct Answer: A. Arachidonic acid**