Final answer:
Lipids are the most common type of molecule in a cell's membrane.
Step-by-step explanation:
The most common type of molecule in a cell's membrane is lipids. Lipids, such as phospholipids, make up the majority of the structure of cell membranes. These lipids form a bilayer that acts as a barrier, enclosing the contents of the cell and controlling what enters and exits.
Learn more about Cell Membrane Composition