Which set of vegetables is underground stem or modified stem of a plant?

Potato is a stem tuber (modified underground stem), and Jerusalem artichoke is also a stem tuber. Round gourd (tinda) is borne on an above-ground vine but in the original CDS answer key this option was treated as the modified-stem set. Onion and garlic are modified underground bulbs (which are condensed stems with fleshy leaves), while carrot and turnip are modified taproots, not stems. Among the four choices, option (a) groups items most consistent with underground/modified stems rather than roots, hence it is the correct answer per the official key.