Step 1: Concept Fatty acids are classified as saturated or unsaturated based on the number of carbon-carbon double bonds.
Step 2: Meaning Linolenic acid is an essential polyunsaturated omega-3 fatty acid with 18 carbons.
Step 3: Analysis Stearic acid (0), Oleic acid (1), Linoleic acid (2), and Linolenic acid (3) show a progression in the number of double bonds.
Step 4: Conclusion Linolenic acid contains exactly three carbon-carbon double bonds in its hydrocarbon chain.
Final Answer: (D)