This is a long-chain polymer carbohydrate that makes up the supporting structure of plants.

The right answer is cellulose

Cellulose is a carbohydrate consisting of a linear chain of D-glucose molecules (between 15 and 15,000). This biopolymer is the main constituent of the plant cell wall, including wood (which is also characterized by a high lignin content).

Pectin is also the structure of plant cells, but they are heteropolysaccharides.