Final answer:
Decreasing temperature decreases membrane fluidity, unsaturated hydrocarbon chains increase membrane fluidity, and cholesterol helps maintain an appropriate level of fluidity in the membrane. Option A.
Step-by-step explanation:
Membrane fluidity can be affected by decreasing temperature, phospholipids with unsaturated hydrocarbon chains, and cholesterol.
Decreasing temperature can compress the fatty acid tails in phospholipids, making the membrane more rigid and less fluid. On the other hand, phospholipids with unsaturated hydrocarbon chains have kinks that push apart the phospholipid tails, increasing fluidity.
Cholesterol, which lies alongside the phospholipids in the membrane, functions as a buffer, preventing low temperatures from inhibiting fluidity and excessive temperatures from increasing fluidity too much.
In summary, decreasing temperature decreases membrane fluidity, unsaturated hydrocarbon chains increase membrane fluidity, and cholesterol helps maintain an appropriate level of fluidity in the membrane. So Option A.