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: None