Answer:
The correct answer is - cholesterol molecules.
Step-by-step explanation:
Cholesterol molecules are the molecules that acts as a bidirectional regulator for the lipid bilayer as it provides fluidity by raising the melting point of the membrane at high temperature and stabilize the membrane.
On the other hand, it also prevents the membrane from clustering or being tough or stiff by intercalates between the phospholipids at low temperatures. Therefore it is the molecule that provides stability to the membrane.