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 into 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: Some proteins consist of two or more polypeptide chain, called subunits (protomer).
The spatial relation of subunits with one another is called the quaternary structure.
Dimers contain two polypeptide chains. Homodimers have same polypeptide chains whereas heterodimers have different polypeptide chains.