Breaking down large hydrocarbons into smaller ones like propane

Steps:

• Fractional distillation separates crude oil into fractions based on boiling points, yielding larger hydrocarbons.
• Larger molecules (e.g., alkanes > C3) need breakdown to form propane (C3H8).
• Identify processes that cleave C-C bonds in hydrocarbons.
• Select the process that converts heavy fractions to lighter gases like propane.

Why D is correct:

• D involves catalytic reforming, which rearranges larger alkanes into branched or smaller chains, including propane, per refining definitions.

Why the others are wrong:

• A. Cracking breaks chains but primarily produces alkenes and gases, not specifically propane without further steps.
• B. Reduction adds hydrogen to unsaturated compounds, not splitting large alkanes.
• C. Polymerisation joins small molecules into larger polymers, opposite of producing propane.

Not enough information on exact D label, but assuming standard refining process.

Final answer: D