What are proteins formed from?

Proteins are indeed formed from polypeptides, which are chains of amino acids linked together by peptide bonds. Each protein is composed of one or more polypeptide chains that fold into specific three-dimensional shapes, allowing them to perform various functions within biological organisms. The sequence of amino acids in a polypeptide determines its unique properties and function, making this option the correct choice.

The other options highlight different biological molecules but do not correctly represent proteins. Single amino acids are the building blocks of proteins but do not form proteins on their own. Nucleotides are the building blocks of nucleic acids, such as DNA and RNA, while simple sugars are the basic units of carbohydrates. Each of these biological macromolecules serves distinct roles in the body but does not directly pertain to the formation of proteins.