Which of the following best describes rhizomes?

Rhizomes are indeed best described as underground stems of plants. This is because rhizomes serve as specialized structures that grow horizontally beneath the soil surface. They are capable of storing nutrients and water, which is essential for the plant's growth and survival. Additionally, rhizomes can produce new shoots and roots, allowing the plant to propagate and spread effectively. This underground growth habit aids in perennation, enabling plants to survive adverse conditions such as drought or harsh winters.

In contrast, seeds are reproductive structures that can develop into a new plant, while leaves are the photosynthetic organs of plants. Roots primarily serve to anchor the plant and absorb water and nutrients but do not have the same growth and storage functions as rhizomes. Thus, the characteristics and functions of rhizomes make the description of them as underground stems the most accurate.