The correct answer is option 2 - stem.

An onion is a modified form of a stem. In onions, the underground part that we commonly refer to as an onion bulb is actually a modified stem called a basal plate. The basal plate produces small, fleshy leaves known as scales. These scales are tightly packed together to form the layers of the onion bulb that we are familiar with.

Onions are examples of bulbs, which are specialized stems that store nutrients and energy for the plant. The main function of the onion bulb is to provide nourishment and support the growth of a new onion plant from the dormant bud located at the center of the bulb.

So, in summary, an onion is a modified form of a stem, specifically a basal plate that produces scales to form the onion bulb.