Final answer:
Among the listed options, lipid is the one that is not a polymer, as it does not consist of repeating monomeric units connected by covalent bonds.
Step-by-step explanation:
The question pertains to identifying which of the listed substances is not a polymer. A polymer is a long molecule composed of chains of units called monomers. Among the given options, proteins are polymers of amino acids, starch is a polymer of glucose units, and DNA is a polymer composed of nucleotides. However, lipids do not form polymer chains; they are not composed of repeating monomeric units connected by covalent bonds like the others. Therefore, the correct answer is d) lipid, which is not a polymer.