What is monomer used in cellulose, a straight chain polysaccharide?

Cellulose is a straight-chain polysaccharide composed exclusively of $\beta$-D-glucose units.

These monomer units are joined together by $\beta$-1,4-glycosidic linkages between the C1 of one glucose unit and the C4 of the adjacent glucose unit.

Thus, the monomer used in cellulose is $\beta$-D-glucose.

Answer: $\beta$-D-glucose