Which type of proteins contain only one amino acid?

The correct choice relates to simple proteins, which are defined as proteins that consist solely of amino acids without any additional components such as carbohydrates or lipids. Simple proteins can exist as single polypeptide chains or even as individual amino acid molecules. They do not undergo significant modification or complexity in their structure.

Understanding the nature of simple proteins helps to clarify their distinction from other types of proteins. For instance, complete proteins contain all essential amino acids and can be either simple or complex, while indispensable proteins are those that the body cannot synthesize and must be obtained from the diet. Complex proteins, on the other hand, are made up of multiple amino acids and may include additional non-peptide components, resulting in a more intricate structure.