Classification — relation type: identify the single pair that represents a part–whole relation while the others do not.

Introduction / Context:
Each pair encodes a relation. Three pairs are “category vs category” (is-a or co-type) relations, while one is a “part–whole” relation. We must isolate the part–whole pair.

Given Data / Assumptions:

• Pistol : Gun → pistol is a kind of gun (is-a).
• Knife : Dagger → dagger is a kind of knife (is-a; order is hypernym→hyponym but still category relation).
• Car : Bus → two vehicle types (co-hyponyms).
• Engine : Train → engine is a part/component of a train (part–whole).

Concept / Approach:
Classify each relation: is-a/co-type vs part–whole. The unique part–whole pair is the odd one.

Step-by-Step Solution:

Verification / Alternative check:
Removing the right-hand item from the left-hand in C breaks the whole (train without engine), which is not true for A, B, or D.

Why Other Options Are Wrong:
They lack the component relationship.

Common Pitfalls:
Letting direction (hypernym→hyponym) distract from the broader category-vs-category grouping.

Final Answer:
Engine : Train