Select incorrect statement, regarding chemical structure of insulin.

Option (3) is the answer because mature insulin does not contain C-peptide chain which is removed during maturation of pro-insulin into insulin. So, it contains only two polypeptide chains $\mathrm{A}$ and $\mathrm{B}$. Option (1) is not the answer because C-peptide is absent in mature insulin molecule. Option (2) is not the answer because polypeptide chains A and B are linked disulphide bridges. Option (4) is not the answer because an extra stretch C-peptide is not present in mature insulin but present in pro-insulin.