What Is Meant by the "Quaternary Structure" of a Protein

What is meant by the "quaternary structure" of a protein?

A) It refers to the four atoms that make up the peptide bonds that join the amino acids together in the protein chain.
B) It describes the unique sequence of four amino acids in the protein chain.
C) It describes the four regular,repeating structures found in the protein chain.
D) It describes the overall three dimensional shape of a protein.
E) It describes the association of two or more polypeptide chains to form one functioning protein unit.