Biological Molecules with Multiple C=C Double Bonds

Steps:

• Identify saturated fatty acid: hydrocarbon chain with only C-C single bonds, zero double bonds.
• Analyze haemoglobin: protein with heme group featuring a porphyrin ring containing at least four conjugated C=C double bonds.
• Analyze collagen: fibrous protein incorporating aromatic amino acids (e.g., tyrosine) with benzene rings, each having three C=C double bonds.
• Compare options: select those excluding saturated fatty acid, as it lacks double bonds.

Why C is correct:

• Haemoglobin and collagen both contain structural elements (heme and aromatic rings) with at least three C=C double bonds, per organic chemistry definitions of unsaturation.

Why the others are wrong:

• A and B include saturated fatty acid, which has no double bonds by definition (fully hydrogenated chain).
• D includes saturated fatty acid (zero double bonds) and omits collagen.

Final answer: C