The structure in a plant cell that provides stiffness is the cell wall.

Here's why:

* Cellulose: The cell wall is primarily composed of cellulose, a strong and rigid polysaccharide. This structure gives the plant cell its shape and provides structural support.

* Rigidity: The cell wall's rigidity allows plants to stand upright and maintain their shape, even against gravity.

* Protection: The cell wall also protects the delicate cell membrane and internal structures from damage.

While other structures like the vacuole play a role in plant cell function, it's the cell wall that is solely responsible for providing stiffness.