Which of the following increases the strength of the hydrophobic interactions in lipid bilayers, and thus makes them less permeable to polar molecules?
a. the presence of double bonds
b. increasing temperature
c. increasing length of the hydrocarbon chains

Option (C).

Explanation:

The plasma membrane of the eukaryotes are made of the phospholipid bilayer and the proteins are embedded or span the membrane bilayer. The carbohydrates are attached in moieties with the protein and lipid.

The fluidity of the membrane depends on the saturation, cholestrol and the hydrocarbon chains. The hydrocarbon chain is non polar in nature that allows the diffusion of non polar solutes across the membrane. This decreases the permeability of the polar molecules and the hydrophobic interactions in the membrane.

Thus, the correct answer is option (C).