The residual solids from the fractional distillation of petroleum are used as

Correct answer: coatings of pipes.

Explanation: The residual solids from fractional distillation (e.g., bitumen/asphalt, petroleum pitch) are not suitable as motor or jet fuels. They are used for protective applications such as coating pipes, roofing, and road surfacing because they are viscous, water-resistant, and adhere well.

• Coatings of pipes — Correct. Bitumen/pitch are common anti-corrosion coatings.
• Raw materials for the cracking process — Cracking typically uses gas oils (heavy liquids), not the solid residue.
• Fuel for the driving tractors — Tractors use diesel, a middle distillate, not residues.
• Fuel for jet engines — Jets use kerosene/ATF, not residues.