An onion bulb is a modified plant stem and is considered a plant organ, comprised of different tissues (e.g., epidermal tissue, storage tissue, vascular tissue) working together to perform functions like food storage.

A. Euglena and D. Paramecium are both unicellular organisms, meaning their entire existence is at the cellular level of organization. They do not have specialized tissues or organs.

B. Hydra is a simple multicellular organism that exhibits the tissue level of organization. It possesses tissues, but not true organs.