Classification — which group contains only birds (the single fully homogeneous “birds” set)?

Introduction / Context:
Another classic classification task: find the only option in which all listed items belong to the same, unambiguous category. Here, we look for the set consisting purely of birds.

Given Data / Assumptions:

• Birds: animals that are avian (e.g., ostrich, hen, swan, duck).
• Mixed sets are to be rejected; the answer must be a homogeneous bird trio.

Concept / Approach:
Scan each group: if any non-bird appears, discard the group. The presence of mammals, plants, countries, or artifacts disqualifies the option as an “all birds” set.

Step-by-Step Solution:
1) A: Ostrich (bird), Hen (bird), Swan (bird) ⇒ all birds ⇒ candidate.2) B: Duck (bird), Lion (mammal), Horse (mammal) ⇒ mixed ⇒ reject.3) C: Zebra (mammal), Cabbage (plant), Rose (plant) ⇒ mixed ⇒ reject.4) D: India (country), world (planet/aggregate), mobile (artifact) ⇒ mixed, non-biological ⇒ reject.5) Therefore A is the only fully avian set.

Verification / Alternative check:
Ask: could all three be found in a “bird sanctuary” list? For A, yes; for B–D, no.

Why Other Options Are Wrong:
Each contains non-bird members, breaking homogeneity.

Common Pitfalls:
Assuming “duck” implies the whole set is birds; always verify all three items.

Final Answer:
Ostrich, Hen, Swan