Which of the following mechanisms is responsible for providing support in plants?

Plants have no bones or muscles, so they hold themselves up in two main ways. Every plant cell is wrapped in a tough cell wall made of cellulose, which gives the cell a firm shape, and in woody stems these walls are thickened with lignin for extra strength.

The second way is turgor pressure. When a plant takes in enough water, the vacuole swells and pushes the cell contents hard against the cell wall. The cell becomes turgid (firm), and millions of firm cells together keep leaves and young stems standing upright. When water is short the cells become flaccid and the plant wilts — clear proof that turgor pressure is doing the supporting.

The other choices belong to animals: muscles and bones form the endoskeleton of vertebrates, an exoskeleton is the hard outer covering of insects and crabs, and the endocrine system makes hormones and gives no support at all.