The smallest among the options provided is B. atoms.
To clarify the relationships:
- Atoms are the basic units of matter and are smaller than cells, molecules, and cell membranes.
- Molecules are made up of two or more atoms.
- Cells are the basic structural and functional units of life and are composed of molecules and cell membranes.
- Cell membranes encase cells and are composed of lipid and protein molecules.
Therefore, atoms are the smallest unit listed.