Which of the following describes the primary structure of a protein?

The primary structure of a protein is defined as the unique sequence of amino acids that are linked together to form a polypeptide chain. This sequence is determined by the genetic code and ultimately dictates how the protein will fold and function. Each amino acid in the chain is connected by peptide bonds, and the specific order in which these amino acids appear is critical to the protein's properties and biological role.

The other aspects mentioned in the choices pertain to different levels of protein structure. For instance, the overall 3D shape of a protein describes its tertiary structure, which is the result of interactions and folding of the polypeptide chain. The arrangement of multiple polypeptides refers to the quaternary structure, which describes how different polypeptide chains interact to form a larger functional protein complex. Lastly, the coiling of the polypeptide is indicative of aspects of secondary structure, such as alpha helices or beta sheets, which are localized folding patterns within the protein. Understanding these distinct levels of structure is essential for grasping how proteins achieve their specific functions.