Presence of Multiple Double Bonds in Biomolecules

Steps:

• Identify unsaturated fatty acid structure: hydrocarbon chain with at least one C=C double bond; polyunsaturated types have two or more.
• Examine collagen: fibrous protein with peptide bonds, each containing a C=O double bond; long chains yield numerous such bonds.
• Analyze haemoglobin: protein with peptide C=O bonds and heme groups featuring conjugated C=C and C=N double bonds.
• Confirm all meet "at least two double bonds" criterion based on structural features.

Why A is correct:

• Unsaturated fatty acids, collagen, and haemoglobin all contain multiple C=C or C=O double bonds as per their biochemical definitions.

Why the others are wrong:

• B excludes haemoglobin, which has heme double bonds and peptide C=O.
• C excludes collagen, which has extensive peptide C=O bonds.
• D excludes unsaturated fatty acid, which has chain C=C bonds.

Final answer: A