Final Answer:
Cholesterol Presence would tend to increase membrane fluidity. Cholesterol acts as a buffer, preventing fatty acid chains from packing too closely together, thereby maintaining optimal fluidity. The answer is D) Cholesterol Presence.
Step-by-step explanation:
The factor that would tend to increase membrane fluidity is Cholesterol Presence. Cholesterol plays a crucial role in modulating the fluidity of cell membranes. It functions as a buffer, preventing the fatty acid chains of phospholipids from packing too closely together. This prevents membranes from becoming too rigid in low temperatures and too fluid in high temperatures.
While longer fatty acid chains (Option A) and saturated fatty acids (Option B) generally contribute to membrane rigidity, lower temperatures (Option C) can decrease fluidity. Cholesterol, however, acts to stabilize and maintain an optimal level of fluidity, making option D the correct choice in promoting membrane flexibility and function.