Cholesterol molecules in the plasma membrane a. form bilayers b. can form spherical vesicles c. have two fatty acid tails d. keep the membrane fluid e. have a polar phosphate head

The correct answer is option d. "Keep the membrane fluid".

Explanation:

Cholesterol is a major constituent of animal cell membranes, being present within its structure in about the same molar amounts as the phospholipids. Cholesterol plays an important role in keeping the plasma membrane fluid. At low temperatures, cholesterol interferes with interactions between fatty acid chains, keeping the membrane fluid and preventing the membrane to freeze. However, at high temperatures cholesterol has the opposite effect and reduces the membrane permeability to small molecules.