Question

Which macromolecule forms a double layer as the primary structure of cell membranes?

Answer 1

Answer;
Phospholipid.

Phospholipid forms a double layer as the primary structure of cell membranes.

Explanation; 
Cell membranes contains of lipids (phospholipids and cholesterol), proteins and carbohydrates groups that are attached to some of the lipids and proteins. 
Phospholipids are composed of a polar head and non-polar (hydrophobic) tail. They line up such that the heads and the tail groups are side by side to form a single layer due to the interactions between the head groups and the tail groups. Two of these layers line up with the hydrophobic tail groups facing each other to prevent the hydrophobic groups away from the polar head groups of other phospholipids and from the water in the surrounding environment. 

Answer 2

Phospholipids form the lipid bilayer that are the basis of cell membranes.