B. atoms is the smallest among the options provided.
Here's a brief explanation of each:
- Atoms are the basic building blocks of matter and generally considered the smallest unit of an element.
- Molecules are made up of two or more atoms bonded together.
- Cells are the basic units of life and are composed of various molecules, including proteins and nucleic acids.
- Cell membranes are structures that surround cells, made up of lipid bilayers and proteins, which are themselves composed of molecules.
So, the correct answer is B.