Final answer:
Phospholipids are the components that account for 30%-40% of the membrane mass and are found in the lipid bilayer, forming the core structure of the plasma membrane.
Step-by-step explanation:
The question is focused on the composition of the plasma membrane, particularly the percentage of the membrane mass contributed by various components within the lipid bilayer. The correct component that accounts for 30%-40% of the membrane mass and is found in the lipid bilayer is phospholipids. Phospholipids form the fundamental structure of the bilayer itself. These molecules consist of a glycerol backbone, two hydrophobic fatty acid tails, and a hydrophilic phosphate group as the head. In addition to phospholipids, other key components of the plasma membrane include proteins, which account for about 50 percent of the composition by mass, and carbohydrates, which are attached to lipids and proteins and make up the remaining 10 percent. Cholesterol is another lipid present in the membrane, providing flexibility.