Cold adaptation increases membrane fluidity

Steps:

• Cold temperatures reduce membrane fluidity, hindering exocytosis by making membranes rigid.
• Saturated phospholipids pack tightly, decreasing fluidity; unsaturated ones have double bonds creating kinks for looser packing.
• Plants adapt by synthesizing more unsaturated phospholipids to maintain fluidity.
• This raises the ratio of unsaturated to saturated phospholipids.

Why D is correct:

• Unsaturated phospholipids lower the membrane's gel-to-liquid crystalline transition temperature, preserving fluidity for processes like exocytosis in cold (principle of lipid packing in fluid mosaic model).

Why the others are wrong:

• A: Decreasing the ratio increases saturation, rigidifying the membrane and impairing exocytosis.
• B: Same as A; reduces unsaturated proportion, worsening cold rigidity.
• C: Phrasing implies increase but omits "phospholipids" specificity; D precisely matches the adaptation.

Final answer: D