An atom is

the largest level of organization in living things

the smallest level of organization in living things

a group of cells that work together

a group of molecules that work together

An atom is the smallest level of organization in living things. Basically, every living thing is made up of cells (according to cell theory). But inside these cells, atoms exist. Of course, as atoms themselves, they are defined by their nuclei (which is composed of protons and neutrons), and compose the different elements.