Molecules with multiple C=C bonds

Steps:

• Recall primary structures: haemoglobin and collagen are proteins with peptide bonds; unsaturated fatty acids are lipids with hydrocarbon chains.
• Identify bonds: proteins feature mostly single bonds in backbones, while unsaturated fatty acids have C=C bonds.
• Check count: unsaturated fatty acids often include polyunsaturated types with ≥2 C=C bonds; proteins lack them in main chains.
• Eliminate options without ≥2 double bonds.

Why D is correct:

• Unsaturated fatty acids contain C=C double bonds in their chains; polyunsaturated variants have at least two, per lipid nomenclature.

Why the others are wrong:

• A. Haemoglobin: protein with single peptide bonds and aromatic rings, but no chain of ≥2 C=C bonds.
• B. Not enough information (option unspecified).
• C. Collagen: fibrous protein with single bonds in triple helix structure.

Final answer: D