A, starch is a polymer compound. When polymer compounds form polymer chains, the length of the chains is different, so polymer compounds are all mixtures, so A is wrong;

B, coal is a complex mixture of organic and inorganic substances, mainly containing carbon, in addition to a small amount of hydrogen, nitrogen, sulfur, oxygen and other elements and inorganic minerals, so B is wrong;

C, the main component of natural gas is methane, but also contains other substances, belonging to the mixture, so C is wrong;

D, acetic acid is a colorless liquid, has a strong pungent smell, pure acetic acid can form an icy solid below 16.6℃, so it is often called glacial acetic acid, and glacial acetic acid is pure, so d is correct;

Therefore, choose: D.