Proteins are linear polymers of amino acids. They fold into compact structures. Sometimes, these folded structures associate to form homo or hetero-dimers. Which of the following refers to this associated form?

Quaternary structure defines the polypeptide composition of a protein and, for an oligomeric protein, the spatial relationships between its protomers or subunits. Monomeric proteins consist of a single polypeptide chain. Dimeric proteins contain two polypeptide chains. Homodimers contain two copies of the same polypeptide chain, while in a heterodimer the polypeptides differ. Ref: Harper 28th Ed. Page-35, Harper 27th Ed. Page: 34