The correct answer is option 'D': High proportion of unsaturated fatty acids.

Explanation:
Membrane lipids of chill-sensitive plants contain a high proportion of unsaturated fatty acids. This is because unsaturated fatty acids have double bonds in their carbon chain, which introduces kinks and bends in the structure. These kinks and bends prevent the fatty acids from packing tightly together, which increases the fluidity of the membrane.

Importance of membrane fluidity:
Membrane fluidity is crucial for the proper functioning of the cell membrane. It allows the membrane to be flexible and dynamic, facilitating various cellular processes such as nutrient uptake, waste removal, and signal transduction.

Adaptation to cold environments:
Chill-sensitive plants, which are not well adapted to cold environments, need to maintain membrane fluidity even at low temperatures. Cold temperatures tend to decrease membrane fluidity, making it more rigid and less functional. To counteract this, chill-sensitive plants have evolved to have a high proportion of unsaturated fatty acids in their membrane lipids.

Effect of unsaturated fatty acids:
The presence of unsaturated fatty acids in the lipid bilayer of the cell membrane prevents the fatty acids from packing tightly together. The double bonds in the unsaturated fatty acids introduce kinks and bends in the carbon chain, disrupting the regular packing of the lipid molecules. This results in a more fluid membrane, even at low temperatures.

Role of fluidity in cold environments:
Maintaining membrane fluidity is essential for the functioning of membrane proteins, including transporters, receptors, and enzymes. In cold environments, a rigid membrane would impede the movement and activity of these proteins. By having a high proportion of unsaturated fatty acids, chill-sensitive plants ensure that their membranes remain fluid and functional even in cold temperatures.

Contrast with chill-tolerant plants:
In contrast, chill-tolerant plants have adapted to cold environments by incorporating a different strategy. They have a high proportion of both saturated and unsaturated fatty acids in their membrane lipids. The presence of saturated fatty acids helps to maintain membrane rigidity at higher temperatures, while the unsaturated fatty acids contribute to membrane fluidity at lower temperatures.

Overall, the high proportion of unsaturated fatty acids in the membrane lipids of chill-sensitive plants is an adaptation that allows them to maintain membrane fluidity and functionality in cold environments.