Here's why:

* Plants: Plants have rigid cell walls made of cellulose that provide structural support, allowing them to stand upright and maintain their shape. This is especially important for plants that grow tall.

* Animals: Animals have skeletons, either internal (endoskeletons) or external (exoskeletons), that provide structural support. These skeletons allow animals to move, maintain their shape, and protect their internal organs.

While other options might be present in both plants and animals, they aren't a fundamental function of both:

* Protection: While both plants and animals have mechanisms for protection, these are often specialized and different. (e.g., thorns in plants vs. fur in animals)

* Movement: While both plants and animals can move, plant movement is usually limited to growth and response to stimuli, while animals have more complex movement systems.

* Reproduction: While both plants and animals reproduce, their mechanisms and processes are distinct.

Let me know if you have any other questions!