The small circles in the protein molecule shown above represent A. Amino acids.
What the small circles represent
The amino acids are the basic units of proteins. These amino acids are linked together by peptide bonds, forming long chains that fold into complex three-dimensional shapes. These shapes determine the protein's function and are necessary for its biological activity.
So, the combination of the amino acids builds complex proteins such as the one demonstrated in the picture above. There are two main classifications of proteins which are globular proteins and fibrous proteins.