Which part of the plasma membrane contributes the most to the fluidity of the membrane?

Phospholipids

Explanation:

The plasma membrane has a lipid bilayer in which proteins are embedded. The chemical composition of phospholipids of the membrane greatly influences the fluidity of the membrane. The length and saturation of lipid tails affect membrane fluidity. Phospholipids with short lipid tails having less than 16 carbon atoms increase the membrane fluidity as compared to the phospholipids with long tails. Phospholipids with saturated fatty acid tails make the membrane rigid while the ones with cis unsaturated fatty acid tails increase the membrane fluidity by their less dense packing.