What type of underground structure do some plants use to store food?

The correct answer is a tuber, which is a type of underground structure that specializes in storing food for the plant. Tubers, such as potatoes, are swollen parts of a stem or root that retain starches and carbohydrates. This storage capability allows the plant to survive adverse conditions and to sprout new growth when conditions are favorable.

Tubers develop as a response to the plant's growth needs, storing energy during times of abundance so the plant can draw from these reserves during periods of dormancy or stress. They can also give rise to new plants, which is a form of vegetative reproduction.

While root systems are crucial for anchoring the plant and absorbing nutrients and water from the soil, they are not specifically adapted for food storage in the same way tubers are. Bulbs, such as onions, serve a similar purpose in food storage but are structured differently, primarily consisting of a fleshy, underground stem surrounded by layers of modified leaves. Rhizomes are also underground stems that can store food, but they predominantly function as a means of vegetative reproduction, aiding in the spreading of the plant rather than focusing solely on energy storage.

Thus, among the choices provided, tubers stand out as the most specialized structure for storing food.