Classification — materials and typical uses: pick the pair that differs because it uses a pure element while the others use alloys.

Introduction / Context:
All four pairs map a material to a common use. Three materials are alloys (steel, bronze, duralumin), whereas one is a pure element (iron). The relation type is the same, but the material class differs for a single pair.

Given Data / Assumptions:

• Steel → alloy (iron + carbon, etc.) used for utensils among many uses.
• Bronze → alloy (copper + tin) used for statues/art.
• Duralumin → alloy (aluminium + copper, magnesium, etc.) used in aircraft structures (spelling corrected).
• Iron → chemical element used for rails (with steel more common today; historically and loosely, “iron rails” is a common phrase).

Concept / Approach:
Group by material class: alloys vs element. The only element-based pair is the outlier.

Step-by-Step Solution:

Verification / Alternative check:
Periodic table vs alloy lists confirm the distinction.

Why Other Options Are Wrong:
They consistently use alloys.

Common Pitfalls:
Thinking “all are material→use so none is odd”; the test often uses a second-level property (here, alloy vs element).

Final Answer:
Iron : Rails