Which of the following describes quaternary (4°) structure in proteins?

The quaternary structure of a protein refers specifically to the arrangement and interaction of multiple polypeptide chains, or subunits, to form a larger, more complex protein structure. This level of organization is crucial for the function of many proteins, as it allows for cooperative interactions between the different subunits, which can enhance the protein's functionality or stability.

In contrast, the other options describe different aspects of protein structure. The linking of amino acids by peptide bonds refers to the primary structure, which involves the linear sequence of amino acids in a single polypeptide chain. The mention of a 3D polypeptide with alpha-helices generally pertains to the secondary structure, which is characterized by local folding patterns such as alpha-helices and beta-sheets. Lastly, the final 3D protein shape indicates the tertiary structure, which describes how a single polypeptide folds into a compact shape due to interactions between its amino acid side chains.

Thus, the correct answer clearly emphasizes the intricacy of multiple polypeptides coming together, highlighting the quaternary structure's role in the higher-level organization of proteins.