Food : Stomach :: Fuel : ? — Maintain the “input → consuming/operating part” relation and pick the correct counterpart.

Introduction / Context:
In “Food : Stomach,” the first term is an input, and the second is the part that processes or consumes that input in a biological system. We must extend the same “input → consuming/operating component” relation to mechanical systems for “Fuel.”

Given Data / Assumptions:

• Food is ingested and processed by the stomach.
• Fuel is supplied to a mechanical subsystem that performs combustion or energy conversion.
• We seek a part (not a whole vehicle) that directly uses fuel.

Concept / Approach:
Fuel is consumed by the engine (internal combustion engine, turbine, etc.) to generate mechanical work. This parallels food consumed by the stomach to support bodily energy needs. Choosing a whole vehicle (automobile, aeroplane) would break the component-level mapping evident in the biological example.

Step-by-Step Solution:
1) Preserve abstraction: input → consuming part.2) Biological: food → stomach.3) Mechanical: fuel → engine.4) Select “Engine.”

Verification / Alternative check:
Any fuel-powered machine contains an engine (or power unit) where the fuel’s chemical energy is released/converted. This is the precise analogue to the bodily organ example.

Why Other Options Are Wrong:

• Automobile/Aeroplane: Whole systems, not the consuming part.
• Rail: Transport mode/infrastructure, not a consuming component.
• Generator: Consumes fuel in some contexts but typically a system; the closer analogue by part-function is “engine.”

Common Pitfalls:
Confusing whole systems with the specific subsystem that performs consumption/processing.

Final Answer:
Engine