This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure and function of protein, Subject name: Biochemistry
Quaternary structure of the Protein 
Select and explain. 
A. The sequence of amino acids in a polypeptide chain, B. The folding of sho (3-30 residue), contiguous segments of polypeptide into geometrically ordered units, C. The assembly of secondary structural units into larger functional units such as the mature polypeptide and its component domains, D. The number and types of polypeptide units of oligomeric proteins and their spatial arrangement
D: The number and types of polypeptide units of oligomeric proteins and their spatial arrangement
Answer: D: The number and types of polypeptide units of oligomeric proteins and their spatial arrangement 
Explanation: FOUR ORDERS OF PROTEIN STRUCTURE:The modular nature of protein synthesis and folding are embodied in the concept of orders of protein structure: Primary structure--the sequence of amino acids in a polypeptide chainSecondary structure--the folding of sho (3-30 residue), contiguous segments of the polypeptide into geometrically ordered unitsTeiary structure--the assembly of secondary structural units into larger functional units such as the mature polypeptide and its component domainsQuaternary structure--the number and types of polypeptide units of oligomeric proteins and their spatial arrangementRef: Harper&;s Biochemistry; 30th edition; Chapter 5; Proteins: Higher Orders of Structure