The correct answer is option 4: Fleshy leaves.

An onion bulb is composed of fleshy leaves that are modified for storing nutrients. The fleshy leaves are the edible part of the onion that we commonly consume. These leaves store carbohydrates and other nutrients, allowing the onion plant to survive during unfavorable conditions like winter.

While options 1, 2, and 3 are incorrect, it is important to note that onions do have a root system, aerial flowers, and aerial stems. However, these parts are not the edible portion of the plant. The roots anchor the plant in the soil, aerial flowers are reproductive structures, and aerial stems support the leaves and flowers. Hence, the correct answer is option 4.