This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Proteins are linear polymers of amino acids. They fold compact structures. Sometimes, these folded structures associate to form homo-or hetero-dimers. Which one of the following refers to this associated form? 
Select and explain. 
A. Denatured state, B. Molecular aggregation, C. Precipitation, D. Quaternary structure
D: Quaternary structure
Answer: D: Quaternary structure 
Explanation: Quaternary structure defines the polypeptide composition and spatial relationship between subunits or protomers of proteins that are assembled from more than one polypeptide units. Monomeric and Dimeric proteins are examples of quaternary structure. Ref: Textbook of Biochemistry By DM Vasudevan, 3rd Edition, Page 129; Harper's Illustrated Biochemistry, 26th Edition, Chapter 5, Page 34