Question

Why is cellulose considered to be a complex carbohydrate?

Answer 1

Cellulose is a carb that has more than two units of sugar linked together and that s why it is considered to be a complex carbohydrate.

Explanation:

Cellulose is a polisacharide `(C_6H_10O_5)_n`. This means it is build of smaller units that are linked together. Those small units are called monomers. The main monmer in cellulose is glucose.
Cellulose consists of several hundred to many thousands of β(1→4) linked D-glucose units a.k.a. monomers.

As seen on the picture below cellulose microfibrils are in a plant cell wall.