Final answer:
The concentration of dissolved solutes on either side of the membrane does NOT influence the fluidity of a lipid bilayer; factors that do affect fluidity are the number of double bonds and the length of hydrocarbon chains.
Step-by-step explanation:
The factor that does NOT influence the fluidity of a lipid bilayer is A. The concentration of dissolved solutes on either side of the membrane. The characteristics that do influence membrane fluidity include B. The number of double bonds in the hydrocarbon chains, where more unsaturated bonds result in more fluidity. Another influencing factor is C. The length of hydrocarbon chains; shorter chains can enhance fluidity. Cholesterol is also a significant factor, as it can reduce fluidity by fitting between the fatty acid tails in the membrane.