The correct answer is: D. tuber
A tuber is a modified underground stem that stores food. It is typically enlarged and fleshy, and has several buds or eyes from which new shoots can grow. Radishes are a type of tuber, along with potatoes, yams, and sweet potatoes.
A bulb is a short, thick underground stem that stores food. It is typically surrounded by fleshy leaves or scales, and has a central bud from which new shoots can grow. Onions, garlic, and tulips are all types of bulbs.
A corm is a short, thick underground stem that stores food. It is typically covered in a papery skin, and has a central bud from which new shoots can grow. Crocuses and gladioli are both types of corms.
A rhizome is a horizontal underground stem that stores food. It is typically long and slender, and has several roots and shoots growing from it. Ginger and turmeric are both types of rhizomes.