Classification (plant parts we eat): Three options are underground storage/organ parts; one option is primarily a leaf/seed herb used above ground. Identify the odd one out.

Introduction / Context:
Botany-based classification often uses the edible plant part. Onion (bulb), beetroot (storage root), and ginger (rhizome) are underground plant parts commonly used as vegetables/spices. Coriander, however, is chiefly used as above-ground leaves (cilantro) and seeds (a spice), not as an underground storage organ. This contrast between subterranean edible parts and an aerial herb yields a clear odd one out.

Given Data / Assumptions:

• Onion → bulb (modified stem/leaf base underground).
• Beetroot → storage root.
• Ginger → rhizome (underground stem).
• Coriander → leaves and seeds used; not an underground storage organ.

Concept / Approach:
Categorize by plant anatomy: underground storage organs versus aerial herb parts. The unique aerial herb in this set is coriander, establishing the odd entry without ambiguity.

Step-by-Step Solution:

Verification / Alternative check:
Market perspective: onion/beetroot/ginger are sold from below-ground harvest; coriander is sold as fresh leaves/bunches and dry seeds.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming “root vegetables” include coriander due to culinary familiarity. Distinguish usage from plant anatomy.

Final Answer:
Coriander