Explanation:

- Cellulose is a complex carbohydrate found in the cell walls of plants.

- It provides structural strength and rigidity to plant cells.

- Option 1: Cellulose – This is the correct answer!

- Xylem is a type of tissue in plants that helps transport water and minerals from the roots to the rest of the plant. It also supports the plant, but it's not a substance; it's a tissue made of several cell types.

- Phloem is another plant tissue responsible for transporting sugars and nutrients. Again, it's a tissue, not a single substance.

- Cytoplasm is the jelly-like substance inside cells where cell activities take place. It doesn't give structural support to the entire plant.