Concept: This question tests your knowledge of plant morphology, specifically identifying the edible part of a common vegetable, Allium cepa (onion).

Why (D) is correct: The edible, swollen part of an onion (Allium cepa) consists of modified, fleshy leaf bases. These leaf bases are attached to a highly condensed, disc-like stem and store food, primarily carbohydrates, which makes them swollen and edible.

Why other options are wrong:

• (A) Aerial stem: The stem of an onion is a highly reduced, disc-like structure, not the swollen edible part.
• (B) Roots: Onions have fibrous roots that grow from the base of the disc-like stem, but these are not the edible, swollen portion.
• (C) Internodes: Due to the highly condensed stem, the internodes in an onion are extremely short and not the primary edible part.

Common Mistake: Students often mistakenly identify the entire onion bulb as a stem, when it is primarily composed of modified leaf bases.

NEET Tip: Remember that many underground storage organs are modified stems (e.g., potato tuber, ginger rhizome), but some are modified roots (e.g., carrot, radish) or leaf bases (e.g., onion, garlic).

Correct Answer: (D)