Which of the following statements about the structure of proteins is true?

a) The sequence of amino acids in a protein represents the primary structure where one amino acid is attached to the other with the peptide bond.

b) The helices of protein are not always left handed. In alpha helix protein, every backbone N−H group donates a hydrogen bond to the backbone C=O group of the amino acid located three or four residues earlier along the protein sequence so it is right-handed.

c) Haemoglobin is an oligoprotein which has four polypeptide chain, two alpha chain, and two beta chain. It has a quaternary structure where each polypeptide has heme group with iron in the center. 

d) Proteins are heteropolymers containing strings of amino acids where the different amino acids are attached to each other by peptide bonds.

So, the correct answer is option D.